I wondered about that too but had not invest into answers but this could explain it:
http://www.ucapps.de/midibox_sid_manual_l.html
SIDn: toggles between LR, L- and -R. Here you can specify, if OSC, FIL and MOD parameter modifications should affect only the left, the right, or both SID channels. The SID number (SID1, SID2, SID3, SID4) has to be selected with the deticated SID buttons.