Pipes always point toward fixtures. For supply pipes, this means that pipes point in the direction of flow. For waste and return pipes, this means that pipes point opposite the direction of flow.

Another way to think of the pipe arrow direction is that pipes always point from big to small.

If you inserted a waste system with pipes pointing in the direction of flow, you can use the “DM Plumbing->Pipes->Reverse Pipe Branch” command to fix it. Select the pipe that all the waste pipes are flowing toward. The command will reverse all the pipes starting at that location.