Dear Hanna,
I heard back from the development team, and they say:
That variable's name seems to be wrong. It should use "max", not "min". The general idea is stated in the remarks:
As for what "exterior angle" this refers to, if you imagine starting with two coplanar triangles that share an edge and then rotating one of them about that edge, and angle in question is the angle through which the triangle was rotated. Specifying a small value for MinExternalAngleBetweenTriangles should therefore help ensure that the triangulation does not have very "sharp bends". The various controls interact in complex ways, and they are treated heuristically, so it's best to think of them as providing qualitative controls on the nature of the triangulation.
The range of allowed values for MinExternalAngleBetweenTriangles doesn't make much sense either (someone used the validator for lengths), but any positive value is allowed, though large values are effectively meaningless. I'm not sure offhand what effect a very small positive value would have. I suppose it would either be treated as a not-very-small positive value or it would lead to an excessive number of triangles.
I filed a development ticket REVIT-166244 to improve the description of that variable.
Thanks for raising this issue!
Best regards,
Jeremy