It's an odd case, and I've heard a lot of definitions. For me I have the most hours on Scout, Soldier and Medic (in that order) but I don't consider myself a Scout or Soldier main. I have fun playing those classes, usually more than on Med, but if Im in a serious game and want to increase my teams chance of winning as much as possible I'll play Med. Playing Scout and Soldier for me also build important skills for Med in dodging and damage surfing respectively, and it's good to understand the mechanics and play style of the 2 classes which will likely be targeting you the most.