The octave up and down buttons (+ and - buttons) change the octave offset of the keyboard. To reset the keyboard to its default Octave (+/- 0) press both Octave up and down buttons together.
Keyboard zones can have additional or independent octaves applied. See Using SL MkIII's keyboard Zones for details.
To transpose the keyboard MIDI notes in semitones, hold Shift and press the Octave Up or Down buttons. Hold Shift and press both the Octave Up and Down buttons to reset the transpose.
Keyboard Zones can have independent transposition applied. See Using SL MkIII's keyboard Zones for details.