There is a similar thread on this topic. I've got a new LG WT1101CW top load washer and our drain pipe can't keep up with the flowrate coming from the washer. It's a great idea to add a larger diameter tube to the drain pipe to allow the excess water time to drain away.
Is there any significance to the second 2" pipe at the outlet side of the trap? Or is this just another drain pipe coming from "upstairs"?
Is there any significance to the second 2" pipe at the outlet side of the trap?
No. The vertical pipe downstream of the trap is either another drain coming in from above or a vent stack. It isn't involved with the capability of the 3" section to provide some surge capacity for the draining washer.
(Well, I suppose it does add a little bit of capacity, insofar as water can fill that drain/vent pipe up to the level of the opening at the top of the 3" section. So that's a little bit of surge capacity. But it isn't its primary purpose.)
