[M100] What is Rex#?

[Hiraghm]

I'm sorry if this has already been answered, but I can't find 
information on it on the website or here, though I'm certain it's been 
mentioned before...

What's the Rex# compared to the RexCPM and original Rex?

I want to decide if I want to wait for a Rex# or get a RexCPM... I'm not 
very into CPM, so I'd be mostly interested in its capabilities in M100 
mode. Which may mean I'd prefer a Rex#?