Websites in Marketplace have no region exclusions (the field exists for compatibility with apps atm but is not used). For phase 0, they are always shown everywhere, but should be higher up in the results if the user region matches a list of regions that we have determined the website is most relevant in. Later for phase 1 we'll allow users to filter on that field to *only* show apps that are relevant in the specified regions. The technical proposal for now is to implement a new field in MySQL/ES, called "preferred_regions", which would contain that list of region. Then ES filtering code would then boost the score of the site when the region matches, using a should query.
This has an impact on bug 1161857 and bug 1161858, so keep that in mind.