Gurgling alone
A gurgle can involve trap-seal or vent pressure, but a sound at one fixture caused by use of a different fixture also supports a shared downstream path. DrainPath keeps both possibilities open instead of calling it a confirmed vent problem or a confirmed main-line clog.
Rising water or return flow
Water rising in a tub, shower, floor drain, standpipe, or toilet after another fixture is used is stronger evidence of downstream restriction. Stop unnecessary drain discharge and arrange prompt review, especially when the reacting opening is on the lowest occupied level.
The lowest opening matters, but does not prove topology
Low fixtures often show a backup first. A sewage ejector or other pump can change the effective drainage path, so “basement” does not automatically mean a gravity building drain. The checker asks about automatic drain equipment before making a wider-scope inference.
Useful wording when you call
Name the source and target: “When the upstairs shower ran, water rose in the basement floor drain.” That is more useful than “the whole sewer is clogged” because it preserves the observation without asserting an unconfirmed cause.
Review the official shared-fixture evidence and thetrap and vent evidence boundary.