I think they're similar, yeah but I'm after the one which has Z-depth information that Combustion can recognize. It's a cool feature where you can place any footage in your rendered 3d scene without having to double-up layers and mask anything out. Unfortunately I don't think Combustion recognizes .IFF's Z-depth information.
I talked to a guy representing Combustion and he said that Alias/Wavefront was in negotiation with Discreet to have Maya be able to Render .RPF's, but that's gomna be a while yet, maybe version 5.0. He did say that there was a plug-in somewhere out there that can make it happen now though.