Slow drains, gurgling, smells: main line or one fixture?
Most pages on this subject hand you a list of seven warning signs and leave you to guess which ones matter. The useful question is narrower: does this pattern point at the main line, or at one fixture? That is what decides who you call and what it costs.
What is happening?
Tick everything that applies. The combination is what carries the information: one slow sink says almost nothing, a slow sink and a gurgling shower on the same floor says a great deal.
The one distinction worth understanding
Everything in a building drains into one line that runs to the street or to a tank. A problem above that point affects one fixture. A problem in the line itself affects everything upstream of it, and it affects the lowest things first, because water finds the lowest exit it can.
That single fact explains almost every sign on the list. A basement shower that backs up while the upstairs bathroom is fine is not a coincidence, it is gravity. A shower that gurgles when a toilet is flushed is two fixtures sharing a restricted path. And a blockage that returns after clearing is the clearest sign of all, because clearing removes what is blocking the pipe without touching what is causing it.
The three that people search for most
A slow draining toilet. On its own, with everything else in the house behaving, the restriction is usually close to the toilet and it is ordinary plumbing work. The version that matters is a slow draining toilet with something else joining in: a bath that empties slowly on the same floor, or a shower that gurgles when the toilet is flushed. That is two fixtures sharing a restricted path, and the path is downstream of both.
Sewage backing up. This is the one that is not a research question. Waste water in living space is a health matter first, and the right move is to get somebody out, keep people away from it and stop running water. It also almost always means the main line rather than a fixture, which matters afterwards: clearing the blockage without finding out what caused it buys time rather than solving anything.
A sewer smell in the house. The common and harmless cause is a trap that has dried out in a room nobody uses, and running the tap for a minute settles that one for good. A smell that persists after every trap in the building has been refilled is a different question, and one that belongs with a camera rather than with air freshener.
Why a clearing that works is not always good news
A rooter or a jet will usually restore flow, and it should: a blocked line has to be opened. The question is what happens next. If roots came up on the cable, they came from a joint that is open enough to let them in, and they grow back. If the line has a sag, whatever settles in it will settle again. If a section has collapsed, the clearing has bought weeks.
So the sentence worth insisting on is simple. Ask what was found, not just whether it is flowing. A contractor who cleared roots and cannot tell you where they entered has cleared a symptom.
What the camera does that nothing else does
It replaces every guess on this page with a distance and a picture. It says where the problem is, how far from the building, what the pipe is made of, and whether the defect is something a liner can cover or something that has to be dug. Nothing else answers those questions, and every price you are given before it is an estimate of an unknown pipe.
What a survey has to produce, and why you should keep the recording, is on inspection and assessment. What the repair then costs, with the published ranges and who published them, is on replacement cost. And what to require of whoever does the work is on the scope builder.