Brake booster?

On my 89 daytona shelby from the brake booster I have a vac line going to intake, to heater controls, and one other short line. I dont know what the third line controls. Can you help me out?

On my 2.2 TurboI, the brake booster has those two lines, and the other goes off to the cruise control module...

[Mike B.]

If you don't have cruise control I think that third port is supposed to be capped off--or you could use it for a boost gauge, or whatever.