I'm not sure of the rules, but as long as the categories are a good match for the domain's traffic everyone should be happy. The surfer will be happy because they are getting "useful links" related to what they are looking for, the advertiser should be happy because they are getting traffic related to their campaign, and you + the parking company will be happy because the landings are translating into clicks.
Case in point, acquired a domain called BlackCoffee in a drop auction. When I had it parked using keywords related to the well known beverage I found it was getting traffic but no clicks, so I investigated & discovered that it had formerly been a fan site for the pop group Blink 182. Now hosted, with content related to the band, it makes a small amount of revenue (albeit, not from parking).
I like to test out my new domains on NameDrive, and when I get a domain with a low CTR I have a hard look at the user searches & other information available. If I can see that the searches are unrelated to my keyword & think I know what the visitors are after I change the searches to match. So far I haven't had another domain as badly mismatched as BlackCoffee, but when I do I'll certainly be trying to use the traffic, not the name to generate revenue.