Inter should mean outside of conference...like interstate = between states. Intra is inside. Grammar lesson over!
In D1, when battling your conference mate could mean costing yourself in post season cash, lower prestige, no shot at the best players, its always a decision to be made to battle a conference mate. As others have said, there is no rule, but for obvious reasons, you should consider if screwing over a conference mate is worth it.
In D2, and D3, there's no such distinction. There is zero conference loyalty in the lower divisions.