Under "Constructing a Battle Force" in the Battle Value document, the section on TAG has this to say:
"If any unit in the battle force is equipped with TAG, Light TAG or a C3 Master Computer, add BV equal to the BV of each
ton of semi-guided LRM ammunition carried (use the ammo BV for the appropriate-size LRM launcher). If the only such piece of equipment in the force is rear-mounted, add half the BV instead."
As in my physical copy, this doesn't actually ever say what this extra BV should be added
to -- the equipped unit, the one(s) carrying the ammunition in question, the force as a whole, or whatever. That information simply isn't there.
If the recent reply from Paul in
this thread on the rules forum is on the level, the wording should probably run along lines similar to "If any units in the battle force are equipped with TAG, Light TAG or a C3 Master Computer, add BV equal to the BV of each ton of semi-guided LRM ammunition carried by the force (use the ammo BV for the appropriate-size LRM launcher) to each of those units. If the only such piece of equipment on a unit is rear-mounted, add half the ammunition BV instead." And even if that should turn out not to be the actual intent, precisely what is could stand to be cleared up.