There is no conflict. There are the following scenarios:
1) You are using the rules for Standard Artillery. In this case, if your spotter for an artillery attack has TAG, reduce the target number by 1 if the target is within range to use the TAG. There is no attack roll required. If you have a unit that is not spotting for an attack, it can use TAG as normal. The spotting unit may use their TAG to make a special attack to designate another target that turn, but doing so will make the artillery attack worse to the same level as not having TAG in the first place. Since that's both a) strictly worse than just using another unit to spot and using the TAG spotter to shoot something else and b) doesn't actually change any net numbers to the attack, there's no point in prohibiting it by the rules.
2) You are using the rules for Advanced Artillery (the original rules published in the book prior to errata). In this case, in which case TAG does nothing on its own, but may be used with Homing Arrow IV.
That's it. It's less complicated than people are trying to make it.