In S3M/IT, 00 means a shared value (or better said: use whatever was used above).
Now ofcource, this doesn't get up for panning and volume though, only for sliding values, not values that set it.
I opened old Impulse Tracker for this, and the test results are shocking.
In Impulse Tracker, the panning is saved in a S3M file from range X00 to XFF too!
That made me even more curious, so I downloaded Scream Tracker 3.21 to see how it is being handled there.
Scream Tracker 3 has NO support for the X?? effect. It says: X?? is an old Amiga (8xx) effect and is not being used.) Then again, there are only a few panning options in S3M. For instance S8x (gus panning) is there which only has 16 values, and SAx is disabled.