I own a REL Britiania B2 Sub, The Mode selector switch changes the Phase from 0 to 180. If your using the REL SPKON (high level) connection (which you should be) Then you would only have to concern your self with the LINE (high level) setting ...you either use #1 for 0 phase or #3 for 180. ( when setting phase the louder one is the correct one).
P.S. even though this sub has not been produced in quite some time, it still is one of the best subs REL ever made, however over time the woofer will degrade and if you call REL in Berkley CA Im sure they would sell you the upgraded woofer for the B2...I think I paid around $350 for the new replacement...well worth it:-)
Matt M