Watershed types are based on the type of drain target the watershed has.
A drain target is the location where the water flow either stops or leaves the surface. Water that flows along an area or across a surface triangle eventually flows off the surface, or it reaches a point from which there is no downhill direction.
If the downhill end of a channel edge is on the surface boundary, then water flowing through that channel continues off the surface. The boundary point is the lowest end of the channel.
If an edge on the surface boundary belongs to a triangle that slopes down toward that edge, then water flows off the surface all along that edge. A boundary segment is a connected sequence of such edges.
If a point is at a lower elevation than all its neighboring TIN points, then when water flows to it, it has no downhill place to go. Similarly, a connected set of points that are at the same elevation and all of whose neighbors are at a higher elevation, is a single drain target. A depression is any such set of points.
A flat area watershed is a flat area, from which water could flow down to more than one drain target. It also includes the parts of the surface that drain to that flat area. A flat area is a connected set of triangles all of whose vertices have the same elevation. Flat areas abut parts of the surface that slopes downhill.
One type of ambiguous watershed is called a multi-drain or split channel watershed.
A multi-drain notch watershed occurs where there is a a flat edge between two points on a surface.