Most Helpful Guy
I would guess that it's something more exclusive and intimate than a hookup or booty call but it's not very serious, there isn't a lot of pressure or planning and nobody is in a rush for anything more.
Most Helpful Girl
The level of commitment i suppose. I say a serious relationship entails making plans, decisions together, introducing each other to family. In semi-casual relationships, there's no planning for the future, decisions don't have to involve the other person and of course there's no need to meet each other's family. I also suppose the defining factor is if each person is only exclusively seeing each other or allowed to see other people.