The primary supertype setting is enabled if a subtype has two or more parent supertypes (i.e. multiple inheritance). In this situation, one supertype (the first added) is automatically set as the primary supertype (although it can be changed via the "Supertype" tab on the "Database Properties" window for the given subtype).

A primary supertype is displayed via a solid line while the non-primary supertype line(s) is/are dashed.

If you have set the "Map to separate tables" option (via the "Supertype" tab on the "Database Properties" window) for the related subtypes and supertypes, the choice of the primary supertype will dictate which parent table the subtype table is a category of. This is because SQL databases typically do not support multiple inheritance. However, from a physical perspective, this setting will make little difference as a 1:1 relationship will be typically created between a supertype and all of its parent tables.

The only exception to that last statement appears to be when the subtypes have different reference modes from their ultimate parent. In this case, Visio exhibits what appears to be a bug that I’ll describe in a later post.