R71 and R72 appear to control which model of EX the DM (XS758) board thinks it is in.

EX5 - R71 installed, R72 not installed
EX5R - R71 not installed, R72 installed
EX7 - R71 installed, R72 installed

There may be other jumpers present or absent depending on whether a particular
circuit element is present but this would appear to be how the firmware decides which
model it is in and therefore which options to allow/disallow.

STOP PRESS: 26th April 2014

There are also five jumpers in play to determine the EX type - J7, J8, J9, J10. They are
located on the pattern side of the DM board in the lower right quadrant. The values are:

EX5 J7 Present J8 Absent J9 Present J10 Absent
EX7 J7 Absent J8 Present J9 Absent J10 Present
EX5R J7 Present J8 Absent J9 Present J10 Absent

J6 also plays a part:

This page © 2005 James Attfield. Excerpts from diagrams © Yamaha Corp.