So transactional immunity actually IS complete immunity from prosecution. (I know- it's weird because it makes it sound like it's a "lesser" immunity) Use immunity is possible, but I'm not sure that's likely given how much other stuff is going on right now. (There's not a lot of incentive for Weisselberg to agree to use immunity under the circumstances)
One of the things that has been driving me crazy about the reporting on this is everyone keeps saying he got immunity "In the Cohen case". But the thing is- the Cohen case clearly implicates Trump Org in a wide variety of crimes- so "Cohen case" is sort of a nebulous concept right now. Does that just mean Cohen's part of the SDNY Case? Does that count all of the Trump Org pieces too?
The Trump Foundation case also directly implicates Weisselberg- literally in the indictment. He had his hands in EVERYTHING
It's totally possible that these are all being looked at separately- they're definitely being tried by different jurisdictions at this point. But in Federal plea bargain deals, for example, it's typical to include a cooperation clause that applies to local/state investigations if there are any. So it would actually be really strange for SDNY to grant immunity to this one thing and not be thinking about what's going on with the Manhatten DA and the New York State AG office.
All of this honestly gets into the practical pieces of investigations, which tend to go beyond my academic knowledge of law. But if were to make a stab at what's going on, I would say that there's a larger plea deal Weisselberg is involved in that includes some culpability for him and also a lot of cooperation.
But that's just a guess right now.