Sorry, should have been a little clearer. Technically, a box stock G23 will be in SSP. Putting in a extended release or a steel guide rod will put you in ESP.
The G35 is technically in SSP, since the extended releases are factory installed parts. Put in a steel guide rod and you have an ESP gun.
Something else that isn't especially clear is that frame mods for the Glock, such as Robar's grip reduction, will put an otherwise stock Glock into ESP. Cutting cocking serations on the front of the slide will do the same.
Most other guns are more cut and dried when it comes to the divisions. Glocks are easily accessorized and you can easily move into another division without knowing it. Not that it's that big a deal in the normal weekend match, but it can become an issue for classifiers, state matches, etc.