Hints are a way to provide extra data to a notification server that the server may be able to make use of.
Neither clients nor notification servers are required to support any hints. Both sides should assume that hints are not passed, and should ignore any hints they do not understand.
Third parties, when defining their own hints, should discuss the possibility of standardizing on the hint with other parties, preferably in a place such as the xdg mailing list at freedesktop.org. If it warrants a standard, it will be added to the table above. If no consensus is reached, the hint name should be in the form of "x-vendor-name."