The expression is often used in analytic philosophy to indicate an idea, an entity, or a reality which cannot be reduced to a lower concept or included in a higher concept.
In the taxonomical structure "genus → species", a species is described as sui generis if its genus was created to classify it (i.e., its uniqueness at the time of classification merited the creation of a new genus, the sole member of which was initially the sui generis species). A species that is the sole extant member of its genus (e.g. the Homo genus) is not necessarily sui generis: extinctions have eliminated other species of that genus.
In law, it is a term of art used to identify a legal classification that exists independently of other categorizations because of its singularity or due to the specific creation of an entitlement or obligation. For example, a court's contempt powers arise sui generis and not from statute or rule. The New York Court of Appeals has used the term in describing cooperative apartment corporations, mostly because this form of housing is considered real property for some purposes and personal property for other purposes.
When citing cases and other authorities, lawyers and judges may refer to "a sui generis case", or "a sui generis authority", meaning it is a special one confined to its own facts, and therefore may not be of broader application.
In statutory interpretation, it refers to the problem of giving meaning to groups of words where one of the words is ambiguous or inherently unclear.
For example, in Road Traffic law, a statute may require consideration of large vehicles separately from other vehicles. The word "large" is ambiguous per se, but may be considered to be "heavy". Thus the relevant legislation will (in Australian law) contain a section "Terms used" or "Definitions" which will itemise all words considered ambiguous, and confer very specific interpretations consistent with natural language. So in this list we find the entry "heavy vehicle means a vehicle with a GVM of more than 4.5 t;" and referring upwards we find "GVM (which stands for “gross vehicle mass”), in relation to a vehicle, means the maximum loaded mass of the vehicle —" with further expansions to cover various contingencies. Thus the term "large" is made equivalent to "heavy" and is (for this purpose) clearly defined, sui generis.
In British town planning law, such as the Town and Country Planning (Use Classes) Order 1987, many common types of land use are classified in "use classes". Change of use of land within a use class does not require planning permission; however, changing between use classes might not require planning permission, but not if the use class is sui generis.
Examples of sui generis transference include embassies, theatres, amusement arcades, laundrettes, taxi or vehicle hire businesses, petrol filling stations, scrapyards, nightclubs, motor car showrooms, retail warehouses, clubs and hostels.
The grant of private hire vehicle (taxicab) operators licences by local authorities frequently has a condition attached that the appropriate sui generis change of use planning permission is granted to those premises to ensure those businesses cannot trade lawfully without the appropriate planning consents.
Even qualified and experienced town planners misconceive that changing use from an existing use class to one which is sui generis always requires planning permission; it does not because the property is not transferred between two existing use classes. Permission is only required if the sui generis use is materially different from the existing one, such as from a petrol station where petrol tanks might have leaked. As in other applications of the phrase sui generis, the decisions will be a unique matter of fact, degree, and professional opinion.
The motto "Sui Generis" has been adopted by the Akitsiraq Law School because it is a sui generis (aboriginal) title in all of Canadian aboriginal law institutes by dint of its title being Inuktitut, the Aboriginal language of the Inuit in the far north of Canada. More importantly, in aboriginal professional legal education, the work of Aboriginal people to define and create contemporary aboriginal education is a thing of its own kind having sui generis admissions and sui generis curriculum.
Generally speaking, protection for intellectual property extends to intellectual creations in order to incentivize innovation, and depends upon the nature of the work and its "characteristics". The main types of intellectual property law are: copyright, which protects creative works; patent, which protects invention; trade secret, which protects information not generally known or readily ascertainable that is valuable to the secret holder; and trademark, which protects branding and other exclusive properties of products and services. Any matter that meets such criteria is protected.
However, sui generis statutes exist in many countries that extend intellectual property protection to matter that does not meet characteristic definitions: integrated circuit layouts, ship hull designs, fashion designs in France, databases, or plant varieties require sui generis statutes because of their unique characteristics. The United States, Japan, and many EU countries protect the topography of semiconductor chips and integrated circuits under sui generis laws, which borrow some aspects from patent or copyright law. In the U.S. this sui generis law is known as the Semiconductor Chip Protection Act of 1984.
In political philosophy, the unparalleled development of the European Union as compared to other international organizations has led to its designation as a sui generis geopolitical entity. The legal nature of the EU is widely debated because its mixture of intergovernmental and supranational elements causes it to share characteristics with both confederal and federal entities. It is generally considered more than a confederation but less than a federation, thus being appropriately classified as an instance of neither political form.
Compared to other international organizations, the EU is often considered "sui generis" because its legal system comprehensively rejects any use of retaliatory sanctions by one member state against another.
A similar case which has led to the use of the label sui generis is the unique relationship between France and New Caledonia because the legal status of New Caledonia can aptly be said to lie "somewhere between an overseas collectivity and a sovereign nation"; although other examples of such a status for other disputed or dependent territories may exist, this arrangement is unique within the French Republic.
In local government, a sui generis entity is one which does not fit with the general scheme of local governance of a country. For example in England, the City of London and the Isles of Scilly are the two sui generis localities, as their forms of local government are both (for historical or geographical reasons) very different from those of elsewhere in the country. Therefore The City of London and the Isles of Scilly are said to be sui generis authorities, pre-dating recent reforms of local government. The Joint Council of Municipalities of Croatia is a sui generis council of municipalities because it was formed after international agreement and therefore has no similar example in the rest of the country.
The legal status of the Holy See has been described as a sui generis entity possessing an international personality.
In a press conference during which reporters were trying to analyse his political personality, Huey Long stated : " . . . say that I am sui generus, and let it go at that. "
In sociology, the sui generis is what has been externalized, then internalized in the overall public and becomes a part of society that simply exists in its construct. It is not something that is not thought to have been created because it is imbedded in everyone's way of thinking and being. Like the idea of love, or going to school, or clothing belonging to a specific gender. These examples are sui generis for they simply exist in society and in all of us without thought of where they come from or how they were created.
A book, movie, television series, or other artistic creation is said to be sui generis when it does not fit into standard genre boundaries. Movie critic Richard Schickel identifies Joe Versus the Volcano as a sui generis movie. Film critic Michael Brooke used the term to describe Fantastic Planet, a 1973 Franco-Czech science fiction film directed by René Laloux.