Applies to: ReMOTE SL, SL MK II
The difference between the 'ReMOTE SL' and 'ReMOTE SL Classic' options in the 'Control Surfaces' section of the Live MIDI/Sync preferences is the way in which the Live instruments and effects are mapped to the encoders and top row of buttons on the left-hand side of the SL.
Ableton implemented a different way of mapping the Live devices for Live 6 so that the mapping would be the same across all controllers supported in Live.
People who used the ReMOTE SL with Live 5 and who prefer the old-style mapping should select 'ReMOTE SL Classic' to retain the original Live 5 mappings. If you are new to using the ReMOTE SL with Live then you can try out both options and see which you prefer.