In the US Military Call Signs are assigned by SIO by date and time frame in some cases. Assigned call signs are usually along the lines of B7E then broken down by person, so the C.O. of Bravo Seven Echo would be B7E 6 or Actual, The X.O. B7E 5 down the line.
They are further broken down into elements 2nd Plt might be B7E 2 as a Plt with the P.C. being B7E-2 Actual or on the company tac Echo Two Actual.
Some units use internal callsigns on the Company tac such as Saber which would make the Company CO Saber Actual, a Plt commander might be Saber 1 actual for first plt, a squad leader in 1st plt might be Saber 1-2 actual (1st Plt 2nd Squad) etc.
Sgt. Rafael Peralta ,United States Marine Corps
Company A, 1st Bn, 3rd Marine Regt, 3rd Marine Divison
We will never forget your valor and sacrifice.
Semper Fi !