Prestige, Artists, and J-Customs are "Team J-Craft," which just means built in the Fujigen Factory in Japan. The J-Customs use to have somewhat wild (for Ibanez) specs, but they've become reasonably standardized lately, typically with Mahogany bodies and bent maple tops. J-Customs have extra fret finishing and maybe use nicer mahogany that the other guitars. I have had many Ibanez guitars and to be honest, I don't notice much of a difference between these guitars other than the J-Customs have very fancy fret end treatment. They all play very very well. I constantly switch between an RG8127 (J-Custom), RG1527M (Prestige) and UV7BK (Artist), and they all play and sound great. The J-Custom probably is my #1, but that may be just the wood combination that I like a bit better.
I think "Iron Label" is just a marketing thing for a set of specs shared across a few different body shapes. Typically black with binding and active pickups. These aren't built in Japan. I recently had a chance to play one, and it seemed fine, but for the price I would just buy a Prestige or used MIJ guitar.