The permanent piece refers to the CP4D practice - it’s intended to create permanent habitat but not be in a permanent easement.
If you Google CP4D, it intends to create permanent wildlife habitat for landowners through this practice since that CRP practice can contain native grasses, shrubs, trees. With those cover types allowed, it’s essentially harder to convert it back to ag land than other CRP practices where you only plant native grasses for example. It doesn’t mean the land will forever be in a permanent easement - CRP program CP4B is for permanent easements.
I did some ChatGPT asking, and it can be in both CRP and CREP. CREP is state specific. Essentially, IL is providing additional incentives for enrolling in CREP. But, it seems CREP might have a bit stronger requirements for the management of the land or longer contract term. Would want to talk to the FSA office about it to get all the facts before possibly buying it, though.