Each callsign has a place in the order . . .
Six is the company/unit leader, actual means you really are talking to that person rather than radio watch or their RTO.
One-Six is the 1st platoon officer.
Two-Six is the 2nd platoon leader.
Five is the company/unit top NCO.
Ten in my experience is the company/unit XO- or officer wearing that hat. In artillery it can also be the FCO.
One-Five is the 1st platoon leader.
When discussing armor, SP guns and SP launcher you use their platoon and then place- Two-Two is Second PLT's 2nd tank/launcher. Three-One would be 3rd PLT's 1st tank/launcher.
When preparing for combat you have a whole LONG list of code words referred to as a COI IIRC which also changes. In combat the unit callsigns should change at the same time you change your radio net's frequencies- so one day your company/battery is referred to as Redbird, then the net shifts and the callsign is changed to Bulldog.
From my experience this is RARELY practiced b/c its not sexy or 'mission related' and breaks down the 'synergy of immersion in the tactical problem' or whatever buzzwords are in style. Just changing each company & battalion's radio nets is challenging enough without being co-located b/c someone never gets the word for the time, has the wrong frequency, the FNG they tell to go change freqs screws up (did not lock the freq, tried to be master, etc) because they are the FNG (see above why a experienced hand does not do it first), reading the wrong page of the COI/SOP or has the wrong version/year of the SOP. Or their encryption gets dumped when trying to shift frequency and the batteries on the encryption loading device are dead since it was left on when you loaded in the motorpool but someone just put it away instead of turning it off. Or your radio drops the time . . . which does not even get into if the amps & antennae are connected properly and your power source is not surging or tripping breakers.
Trust me, getting properly working encrypted radio communications can be a extremely entertaining exercise . . . if you are not the one answering to a senior NCO or officer about why you cannot speak to higher or your subordinate elements. But its not interesting unless you have done it (have some fun stories about a few dumb arses) but it generally not something you plug into the narrative of a story.
Pat, you obviously had some better officers. We had a few that always got saluted in the field . . . here's your clue Sir.