Slot refers to slot 1-10 in the power tray. Tray is power tray 1-9. This allows players to specify which tray to execute a power from, even if it's not visible. Note that if and when you respec, any binds using /powexec_tray will not care what power is there, they will just attempt to execute whatever is in that slot. If you have an Armor there, then respec and place a self-heal in that same slot, that same bind or macro will fire off the self heal.