The widely-used name for this set of theorem and heuristics is known as "The Travelling Salesman Problem", and a little googling will get you literally thousands of results.

MapPoint uses four weighted sets of advanced heuristics based on "allowable" paths. The actual algorithms are propireatary in nature, as is the case with most commercial routing software.

There are a multitude of heuristic algorithms in public domain, however.
I'd suggest a starting point of:
TSPBIB Home Page

HTH
Paul