What does the adapter that is connected to the trottle housing connect to on 1999 Ford Expedition

On the engine it is connected to the PVC tube and to the front Intake what is the other tube facing the rear going to

Asked by for the 1999 Ford Expedition
poss vac port of intake manifold
Does this at all connect to the cooling system
Look at digram under the hood.
Does not have diagram this is why I needed Help
I guess i have just been lucky enough to work on those that do.
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