I have a graph which corresponds to an n by m grid of vertices. Edges in this graph are only allowed to point from one vertex to an adjacent vertex. In this situation, it would be wonderful if I could define an edge, so that its target is not a vertex_descriptor but some other construct which would allow me to store the target in just one byte or less instead of the full unsigned int which I use to describe my vertices. That is, the edge could go from vertex 102 to a descriptor in the range [0,8] which would give you enough information to compute which vertex you're talking about.
The graph I'm working with is rather large. It could have millions of vertices and saving this kind of space would be fantastic.

If you're defining your own graph type, you can define your descriptors however you want. The CSR-graph does something similar, IIRC. That's probably not a very helpful comment. If you can post some code, I might be able to give a better answer.