Clothing Q for IM

I figure I’m going to buy a wetsuit for my first IM, but what do I wear underneath?

Usually for tris I wear just tri-shorts. I herd for longer ones you want to wear a full tri-suit for sun protection.

Would it be out of the norm to just wear trishorts and nothing on top?

Whats the real reason for wearing the full suit vs just the shorts?

I wore tri-shorts and no top for the swim at IM Moo last year. Then I put on a shirt for the bike, and changed socks for the run. I wouldn’t wear a tri suit for anything longer than an Olympic because if you need to stop to pee, it’s more difficult to get out of.