the charcoal canister acts like a storage tank for the fuel vapors from the tank, when it gets to a point, the solenoid flips open allowing the vapors to be consumed by the motor
I thinks thats what it does anyway
then i seen you asked about the fitting itself and not what purging was about...i googled the fitting
Looks like service port for diagnostic
EVAP purge/pressure diagnostic station pressure regulator.
connect the J 41413 EVAP pressure/purge diagnostic station to the EVAP service port, pressurize the EVAP system to 10 in. H2O and observe the Fuel Tank Pressure display on the scan tool while moving connectors and wiring harnesses related to the EVAP purge valve. A sudden change in the display will indicate the location of the malfunction.
Incorrect vacuum line routing.