I just finished a laundry room on the top flooor. The plumber used 2" PVC drain pipe.

Starting from where the washing machine hose will be hooked into (40" from the floor), the 2" follows the following path:
Down through a tiled floor with a T-Y for an aux floor drain, then into a trap, a Tee w/cleanout, a tee follows used for a vent and then to the basement main drain.

My question is; should the vent be before the trap ? What is the vent doing. Venting the main waist line? The laundry room? Or both?

Do I have the wrong concept of what a vent is? I'm confused. Please give me a lesson of how this works. Thank you, john

The vent is always located downstream of the trap. It vents the trap. However, in my area, what you described does not meet code. The trap for the washer drain must be above the floor(meaning the vent must be above the floor) and the floor drain would need its own trap and vent. The purpose of venting is to prevent the siphoning of traps, that's why they need to be located downstream of the traps, they would do absolutely no good before the trap.

Thank you mygirls77 for your answer.
It clears the wrong concept I had about vents.
The flooor trap would have been imposible for my situation and it's too late to change the location of the trap, unless there is a functional problem when I install the washer. If I have a problem, the aux drain can be plugged. Should there be an over flow, the water would be contained in the laundry room because of the way I fashioned and water proofed the floor like a shower pan.
Thanks again,
John