Goes to the vacuum actuator on the 4 port heater control valve.
When vacuum is applied the actuator shuts off the heater core. This is only in the OFF and MAX A/C positions.
When the MAX A/C position is selected the recirc door is closed, front vents are actuated and the heater core is shut off.
Since all of these are on the same vacuum "circuit" a loss in vacuum will cause them all to default. Front vents default to defrost vents, recirc door defaults to open allowing fresh air in, heater core isn't bypassed.
Check out this video I made for visual reference.