These are called a “call sign” [1]. The International Telecommunication Union issues prefixes[2], and in the US the Federal Communications Commission issues four- (formerly three-) letter call signs under their prefixes[3][4] to commercial broadcast stations. They also require (or required? not sure what the modern regulations are) stations to regularly announce their call sign for identification purposes, so a lot of radio stations folded the sign into their branding since they had to use it anyway.

[1]: https://en.wikipedia.org/wiki/Broadcast_call_signs

[2]: https://en.wikipedia.org/wiki/ITU_prefix

[3]: https://en.wikipedia.org/wiki/Call_signs_in_the_United_State...

[4]: Notably, ‘K’ and ‘W’ generally indicate geography, west/east of the Mississippi River respectively.