The XL line used a common motherboard, but was not exclusively limited
to CD-based. With a security key and a CD or HDD swap (and sometimes
more RAM), you can switch among the following:

XL 5000
XL 6000
XL Extreme
XL Gold
XL Platinum
XL Double Platinum (trivia and pictures update)
XL Titanium

Merit then introduced the MAXX line, with a new motherboard, and
changed the naming methodology to use the names of precious gems.
These are primarily HDD-based. If you have a MAXX motherboard, you can
switch among them with a new security key and drive. Some of the
releases:

Emerald
Emerald 2
Ruby
Diamond
Double Diamond
etc.

and then you get into FORCE and RADION...