If the downhill end of a drainage channel edge is on the surface boundary, then water flowing through that drainage channel continues off the surface. The boundary point is the lowest end of the drainage channel.
If a watershed has this type of drain target, it is called a boundary point watershed.
In the following illustration, point p5 is a boundary point, the drain target of the drainage channel p7-p6-p5:
Note: The "t" in the illustration indicates that the triangle was split between two watersheds because the water flowing across that triangle could go to either of two watersheds.