Non-ITU prefix

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 recognized.
They can be:

unofficial - used by individuals without any issued permit
temporary - issued by local authority
official - recognized internationally by different organisations

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.

E8, E9, H5, J9, On, S4, T9*, Un, V9, Xn, YZ*, Z4-Z7, Z9, 4N*.

Unavailable: Under present ITU guidelines the following call sign prefixes shall not be allocated. nn, x0, x1, 0x, 1x, Qx

no prefixes beginning with Q are used—they may be confused with Q codes.
no prefixes with the digits 1 or 0 are used—they may be confused with the letters I or O.
two digit prefixes are not as yet considered by the ITU.