I think
1261 has the closest measure of it and as always the origins are historical. Much of the early aeronautical practice was adopted from the swabbos (Navy) who already had a fine system of signalling by flags, long before Marconi came along.
As WT/RT developed, these signals were kept fairly standard as everyone was already familiar with them and it avoided confusion. I appreciate that this may be a strange concept to more recent joiners of our industry who are accustomed to being bu99ered about by incessant policy changes.
To this day, maritime code flags either represent a letter of the alphabet when in a string or, individually, a specific message. Long before we learned how to send out radio signals, this flag was the letter
C
On its own it means
Yes, Affirmative, Correct, etc. . . . or just
Charlie. It's simply something that's been handed down through the generations that everyone still seems to understand
