Sheer descent
Also called: sheer descent waterfall, waterfall curtain, sheeting water feature.
A sheer descent is a pool water feature: a wide slot that drops a flat sheet of water, the waterfall-curtain look. It is sold by width, but flow is what makes it work, at roughly 12 gallons per minute per linear foot for a clean sheet.
Why it matters
A single-speed residential pump commonly moves somewhere around 60-80 GPM, so one 4-foot unit can ask for most of your filtration flow. For anything wider than about 2 feet, plan a dedicated pump or a valved dedicated line, and get it into the plumbing plan before the shell is poured. It also needs a raised wall to come out of. Builder guidance puts the usable drop at 18 to 36 inches above the waterline. It is the loudest feature of the group and the least-preferred sound in controlled listening, and it cools the pool the most. Like any feature it aerates, which pushes pH up.
In numbers
A 4 ft sheer descent wants about 48 GPM for a basic sheet and up to about 96 GPM for a full curtain. The same requirement gets published as 12 GPM per foot and as 1 GPM per inch, which are the same number.