From the comments I now think I understand the sets VS multiplies difference.
Murder holes sets min range to zero, ballistics sets leading shots or other “aiming” to yes.
Supplies sets the new food cost.
Parthian tactics and thumb ring add to accuracy, change the multiplier on firing speed.
Squires is a multiple on speed applied to all units.
Damage increases are added on to every unit that comes out.
Sicilians unique techs are a good example: Hauberk adds the damage to knight line, this repeats on every unit.
First Crusade only allows the one time add of those Sergeants from TC’s.
If this tech instead let you get 5 upon completion of a castle, it would then be repeatable because you can trigger that benefit again.
Edit: I was wrong.
I was thinking this was kind of an insight into how it was coded, but it turns out it all seems to be for the mods and cheats.