US radio callsigns

I just finished Antal's "Infantry Combat: The Rifle Platoon."  Very
cool, but the callsigns for communication within the platoon weren't
explained (that I noticed-- it didn't detract from the book, FYI).
One-six, one-three, one-four, etc... including Bravo one-six (I
believe Bravo to be the platoon designation...it's been a few days)