spell - The name of the spell that's being casted. (string)
rank - The rank of the spell that's being casted. (string)
lineID - Spell lineID counter. The Nth spell that UNIT_SPELLCAST events have fired for. Unique to each spell cast - UNIT_SPELLCAST_START and UNIT_SPELLCAST_SUCCESS events referring to the same cast will have the same lineID. Resets to 0 when the count reaches 255, but this number may just always be 0 for some spells. (number)
spellID - The id of the spell that's being casted. (number, spellID)