Split-jamb doors are a lot like conventional prehung doors, but they have a two-piece jamb joined with a tongue and groove. The tongue-and-groove joint, which is hidden by the stop, allows both sides of the jamb to be cased at the millwork shop. Then the jamb can be separated when it’s time to install the door.
Not only does the setup eliminate installing casing in the field, but it also gives you some wiggle room with regard to wall thickness. This quality makes split jambs great for old houses, which often have wavy plaster and odd-size studs. In addition, the millwork shop generally installs the casings and prepares the jambs for less than what I’d pay a competent finish carpenter to do the same work.
I’ve heard old-school carpenters deride split-jamb doors, claiming they can’t be shimmed, but that’s simply not the case. You just have to change your methods. I’ve never had a problem with any of the several hundred split-jamb doors I’ve installed over the years.