pls help about team shape

from this forum, i already have known about team shape. but still confused with some parts. in flexible, team divide into three parts ( attack, support and defend ). in fluid, team divide into two parts ( attack and defend ). flexible is quite easy to understand but fluid is too complicated! so, for example, maintaining standard mentality, if i put inside forward with attack duty and full back with support duty in flexible team shape, it means the fullback will support the inside forward by covering and overlapping. but what would actually happen in fluid team shape for same mentality? will the full back be more creative and less defensive minded? what would happen for aggressive minded players like regista/support, ball playing defender/defend, ball winning midfielder/defend etc. for same mentality?