My impression is that when there is more fluidity the individual characteristics of players (eg: traits) emerge more clearly. Which would make sense, the more fluidity the system is, the freer are the players to interpret their roles. I know most people think it is just a label, but to me there makes no much sense. It's not easy to spot, but my impression by watching the game is that everything becomes more fluid when the tactic is fluid, movement, exchange of positions, pressing.... and so on.