There's a need for defining multi-monitor. Depending on the person, it could mean something simple like the game functioning properly if there's a second monitor, to the intermediate step where it won't auto-minimize when you choose to do stuff on the second monitor, and where the game has explicit functionality on secondary monitors.
Without the definition, I feel I might be classing games incorrectly as supporting a second monitor (when it's really just okay with the second monitor.)