A 3 or four letter "call" is issued by the FCC for AM, FM, TV & SW broadcast
stations. K calls are almost always west of Mississippi river & W calls to
the east. We also have A & N calls for other things. In Romania you have
calls starting YO.
The Americas are where they are used often. Examples are
Canada C & VO
Mexico XE
The calls often stand for something. Examples
WLS Worlds Largest Store
WATD We are At The Dump
WARE in Ware Mass.
WTAG Worcester Telegram And Gazette
In US its law to use calls every 1/2 hr IIRC.
BTW airplane tail numbers usually use the same system.
73 Zeno
LFOD !