Water stop is basically a gasket between two layers of concreteTime to show my ignorance. What's a water stop?
There are several types , often a flexible pvc strip is embedded in the first layer of concrete( footing) with half of the strip left protruding from footing that the wall concrete is poured on top of
In this case a flexible roll of betonite impregnated waterstop ( a little like old school automotive window sealant but not as sticky ) was proposed and accepted then installed , then at the last moment the engineers said the wanted a product that was bentonite free
So I had to cut forms open to change it
Hope that gives you an idea