Studio One doesn't use instrument definitions. It uses bank and program numbers in the inspector AND Program number values in the PRV controller pane.
 
My Ensoniq SQ1+ has 2 sound banks: Internal and ROM. It receives program changes using the last 4 MIDI change numbers (124-127) to control Bank changes. ie: After program change 124, programs 000-079 will select Internal sounds. After program change 125, programs 000-079 will select ROM sounds. 126 & 127 are for Card banks.
 
In SO3's Inspector selecting Bank=0 and Program=1-99 only gives me ROM sounds. No Internal or Card sounds. And SO3 ignores my synth's panel selection.
 
So to get Internal sounds I have to select Bank=0, Program=124 for Internal sounds and Bank=0, Prog.=125 for ROM sounds, and so on. Then to pick the actual sound I have to draw a Program Change controller in the PRV's Controller Pane. A controller value of 1 gives me voice 1, etc. Somewhat awkward but it works. A 3rd data entry box for the patch # in the Inspector would alleviate the PRV edit.
 
IOWs, because SO3 ignores the SQ1's panel Bank selection, the Internal and Card sounds require the PRV controller but the ROM sounds don't.

Either way I can use the PRV controller to change the track voice multiple times within the same track which might be handy. Something Sonar can't do AFAIK.