Is the "trap" near the washing machine for the whole house or just the washing machine?

Question

The "trap" being a small round thing that is behind the washer (not the vent to the dryer). My grandpa thinks that something is clogged causeing a leak and wants to know if the "trap" is for the washer or the whole house?

Answer 1

This trap probably leads to the main sewer line but there might be other drains leading into the sewer line after it - like the kitchen drains.  It will probably work as a main cleanout.