Functional English is usage of the English language required to perform a specific function. This is typically taught as a foundation subject as a good command of English is often required for academic study and career progression. In some cases, a particular form of technical English may be required for a particular vocation such as Aviation English. Such specialised usage is known and taught as English for Specific Purposes (ESP).
Functional english clarifying
References
Functional English Wikipedia(Text) CC BY-SA