EVAP purge solenoid valve?

My purge vacuum diagnostic switch has a vacuum line hook-up on each side of it. One goes to evaporator canister and the other goes to purge solenoid valve. There is a third nipple coming out the front side of it.
I do not remember unhooking anything from it when I took engine apart and do not see anything around to hook-up to it.
The nipple is there for a reason. Any ideas what might hook-up onto it? Thanks