There's lots of crossover between drainage engineers and plumbers. Both work with pipes that transport water & waste. And lots of tradespeople have experience and training in both disciplines.
The main difference is that plumbers work with the water flow inside your home. So they're great for tasks like unblocking toilets or showers, or fixing a leak under the kitchen sink.
Drainage specialists tend to work with the drains and pipework outdoors. Tasks like excavation - that might be required to fix large outdoor waste pipes - would need a specialist drainage engineer, rather than a plumber. They can also provide specialist equipment for CCTV drainage surveys, drain jetting & other drainage services.
If you're not sure whether you need a drainage specialist, a plumber, or another tradesperson, that's no problem. We've got tried & trusted local tradespeople in all disciplines. Just let us know about the job and we'll match you with an experienced & vetted tradesperson.