Many architects stay in close contacts with plumbers so they can learn of new changes in codes and regulations. Blueprints are sometimes considered a loose standard as to how the house is put together.
Most contractors will tell you the same thing. Most blueprint plans need to be altered to make things fit together.
Plumbing symbols on blueprints help plumbing contractors know how and where to do pipe layouts and venting systems. They show where to put things like shut-off valves in fresh water supplies and things like clean out plugs in septic lines.
All these things are planned out by a licensed architect who specializes in house design. Architects need to know all the modern building codes in all the areas of architecture, electrical and plumbing systems.
It’s also a good idea to keep the blueprints around once the house is finished so that future plumbers and homeowners know where different plumbing components are.
These blueprints are also important to the carpenters who frame the house. They need to know where to put vents through roofs and special boxes like laundry washer spigots and also they need to know where to frame in access doors to plumbing fixtures.
So all the contractors have to get to know these blueprints so they can all work together. That means they all have to be able to read different kinds of blueprints to some degree. Most of the time, it’s the general contractor that makes sure all sub-contractors are on the same page.