Running trap

I can't find a definition either, which is surprising. My understanding is that it's a trap that has horizontal inlet and outlet openings at roughly the same level, and that doesn't serve any particular fixture. When I used to live back east, I saw them frequently in the form of "whole house" or "sewer" traps. I've never seen that application on the west coast. The "standard design" requirement prohibits people from making up their own traps from spare parts. It works hand-in-hand with the requirements that the trap be listed or labeled and that it be stamped or marked with the manufacturer's name. The IRC simply prohibits building traps (perhaps that's where the "running traps aren't allowed" thing came from), while UPC (1008) says that building traps are prohibited unless required by the AHJ, then it goes on to give specific requirements for installing them. In practice, the only times when I see problems with running traps is at bathtubs or showers, when someone places the traps several feet away from the drain outlets.

source https://www.inspectorsjournal.com/topic/18978-running-trap/?do=findComment&comment=173413