Oh! Um, well it's something that we used back home over radio traffic, to distinguish members of different teams from one another, as well as rank and position. It made it easier to communicate with other branches in emergency situations, since remember everyone's name and other details can be hard in the heat of the moment.
For example, back when I was in Riot Force 6, I was "StarS Two" because I was the second rank in the unit, first ranked Forward. That put me right behind Stars One, Captain Nanoha. The other team was called Lightning, headed by Enforcer Fate.
When someone outside of our branch calls out "Stars One!", they immediately know who they're talking to, even if they don't know their name.
[ Smiling easily. ] Before I came here, my callsign was Swordfish One. It has a nice ring to it, doesn't it?