If you make an item with an AC range effect, does the AC have to be built into the item? or can it be used on any AC?
Well, if you use touch range, the obcet to be touched does not have to be glued to the item, so I would rule: No, it is not necessary to build the AC intothe item.
However, there are circumstances I would strongly reccomend to do so:
1.) Emergency LoHC: This way you have to use the item and do not have to search for the AC as well.
2.) Items to be used by those not educated in the arts of magic. Hell, who known what happens when a grog uses LoHC but uses the wron AC? Can he use any AC at all?
A spell, or spell effect, is not limited to working for only 1 AC (unless the magus specifically designs it that way.)
So, I'd say "No", but how the AC is brought into the equation should be defined in the activation description. If the decision is to only use the AC built in, then that's the effect. Many other options are possible, including building in a "default" AC, but allowing other AC's to be used instead if desired ~and~ if the user knows the proper activation. (Some SG's might decide this effect requires a magnitude of Complexity, or a couple/few levels for same. Don't think I would, but ymmv.)
I was going to make a LoHC item for a mage that was deficient in corpus. It was going to be a jewelry box that leaps to the acrane connection that is inside the box. That way whenever you are in a dangerous situation, hopefully you prepared for it by putting a safe arcane connection in the box...