A vertex is a point on a 3D mesh. Vertices define faces. Vertices and faces make up the ‘mesh’ that is drawn on screen when you’re chatting in IMVU. A boundary is defined as the outer edge of a mesh. So, the vertices that live along the boundaries of a body part mesh are called ‘boundary verts’.
For example, an avatar top product has boundary verts along the neck (where it connects to the head), waist (where it connects to the lower body) and wrists (where it connects to the hands). Each boundary vert needs to be placed in the exact same location as its neighboring body part. What’s more, each boundary vert needs to be weighted the exact same way as its neighboring part. Otherwise, you get nasty gaps where the body part meshes meet.
NOTE: This ‘nasty gaps’ business is actually very important. When new geometry does not work well with existing clothes, customers complain and the overall sales of that Creator are hurt. So, take the time to get your placement and weighting right on your boundary verts. It really matters.
I most 3d editing applications there is a tool or method to “snap” vertices to one another so that they are both in the exact same 3D space.
Making sure the vertices at the edges of meshes are weighted correctly is vital to making an avatar body part or clothing work. Most vertices will need to be weighted 100% to one bone. There are a few cases on the male avatar where vertices need to be weighted 50% to one bone and 50% to another.