Clarify definition of edge vertex

3MF Core Specification.md

@@ -514,7 +514,7 @@ The \<mesh> element is the root of a triangular _mesh_ representation of an obje
 
 If the mesh is under an object of type "model" or "solidsupport", it MUST have:
 
-- Manifold Edges: Every triangle edge in the mesh shares common vertex endpoints with the edge of exactly 1 other triangle.
+- Manifold Edges: Every triangle edge in the mesh shares common vertex endpoints with the edge of exactly 1 other triangle. A vertex endpoint of an edge is defined via the index of a vertex.
 - Consistent Triangle Orientation: Every pair of adjacent triangles within the mesh MUST have the same orientation of the face normal toward the exterior of the mesh, meaning that the order of declaration of the vertices on the shared edge MUST be in the opposite order. For example, if Triangle1 has vertices ABC and Triangle2 has vertices DEF and Triangle1 and Triangle2 share the AB/DE edge, then it MUST be the case that vertices A=E and vertices B=D (see figure 4-1 below). A triangle face normal (for triangle ABC, in that order) throughout this specification is defined as a unit vector in the direction of the vector cross product (B - A) x (C - A). For example, the triangles shown in figure 4-1 have normals pointing out of the mesh.
 - Outward-facing normals: All triangles MUST be oriented with normals that point away from the interior of the object. Meshes with negative volume will not be printed (or will become voids), in accordance with the Positive fill rule defined in the next section. In combination with the preceding two rules, a mesh is therefore a continuous surface without holes, gaps, open edges, or non-orientable surfaces (e.g. Klein bottle).