Selenis was where Qetesh's forces attacked Ba'al. Qetesh apparently won the battle, crippling Ba'al's flagship and killing 10,000 of his best Jaffa. Ba'al still held a grudge over the incident years later when he met up with Qetesh's host, Vala Mal Doran.
Note: when SG-1 visited P8X-412 with Vala, they watched a play renacting when Qetesh defeated Ba'al and "saved" its people (9.05 "The Powers That Be"). It is unclear whether or not this is the same battle. If so, it would imply that Selenis is P8X-412.
--Aurora 19:34, 6 August 2006 (PDT)