The Editor's Guidelines simply say "exceptional" sites. Exactly what that means, gets worked out in the internal forums.
Fortunately, it's not an issue for site suggestions, since the Submittal Policies are very explicit: suggest a site to The One Most-Specific Appropriate Category. During site review, an editor will consider whether other listings are appropriate.
There are a couple of socially-acceptable exceptions to the "suggest to one category only" rule.
(1) A site with content in multiple languages may be suggested to a category in each language.
(2) Sites of organizations with both global relevance and local emphasis may be listed in a topical category as well as in their local category. These would include universities (not high schools), musical ensembles, manufacturers who ship over a large area (but not local-personal-service entities like real estate agents, travel agents, plumbers, restaurants), churches (but not most religious sites), etc.
Because double-suggesting such sites actually helps editors most of the time, nobody will complain and some editors will thank you.
It's important to remember that heavily-listed sites are multiply listed because of editor consensus, not because they were aggressively suggested. Most of the aggressively-suggested sites aren't listable at all, and don't ever GET listed--or if they are listed at all, it's temporarily and accidentally.