I figured this out, so wanted to share, as it was a bit unintuitive. I wanted to record Little People, Big World, but only regular season episodes (IE: NOT specials). The typical way I do that in other software is to specify that the episode has to be greater than 0. Figured it would be easy in JRiver (which ultimately, it was).
In your recording rule, add a rule, and choose custom from the drop down. Then in the box next to Custom, enter [Episode]=>0
Note the '='. I was trying all types of variations on [Episode]>0 with no luck.
Hope this helps someone...or helps me in 2 years when I can't remember how I did it!
Also, note this will only work if your EPG data has episode information.