Prefixes that are used for radio and television are usually allocated by ITU. They also form the basis for, but do not exactly match, aircraft registration identifiers. But in some cases, especially among amateur radio operators other, by ITU unallocated, callsigns are used when operating from disputed areas or countries that are internationally not (yet) recognized. They can be:
Prefixes for those stations are either unallocated or unavailable by ITU definitions.
ITU unallocated and unavailable call sign prefixes
Unallocated: The following call sign prefixes are available for future allocation by the ITU. (x represents any letter; n represents any digit from 2–9.)
(* Indicates a prefix that has recently been returned to the ITU.)
Unavailable: Under present ITU guidelines the following call sign prefixes shall not be allocated . (x represents any letter; n represents any digit from 2–9.)