I thought it was the extra terminal for lamp power (grounding).
Now I see it is a DPDT switch.
An example of my stupidity (in this case resulting from too fast a read/look).
Thanks soundman - again.
And champak, that's 2 of us supporting relays.
If I was too technical, try me again.
But think of a relay as a "switch extension" where any small on-off switch controls the real switch that switches whatever connections and power that you need.