I guess it could mean a few different things: either the fish needs lots of cover (most catfish), or it needs to graze on the organisms/algae/plants that grow in a planted tank (otos). A third reason could be that the fish requires very clean water, such as that in a planted tank where most nutrients are sucked up by the plants (discus/rams).
Regardless, there are ways to get around all of that without live plants by using either artificial cover, feeding fresh vegetables regularly, and changing water very frequently.
But, if you just want to call your tank "planted," You'll need to use only live plants. Otherwise, it's semi-planted at best. I imagine that few people will yell at you for deviating from this, however.