The main difference between the PMX700 and PDX700 is the PMX700 has adjustable horizontal offset. The horizontal offset allows the extension arm to be moved left and right on the wall plate. It does not affect extension or retraction, other than it will change where the TV sits on the mount.
PMX700 also has a tool-less tilt system that allows you to set the tilt on your TV without the use of a socket set.
For more information on the PMX700 mount, and other mounts in the PMX series, visit our PMX PRO SERIES page.