Ultras culture and casual culture have the same aim; to outdo their competitors. Every Ultras group in Europe like Casuals, want to be known as the best there is in their department.
Ultras culture is simple, supporting the team visually with colour/displays/pyro and vocally with raucous noise.
Casual culture in my opinion, is the bragging rights outside the ground. Who’s more feared? What mob has done over another mob on their own patch? The subculture of the casual scene used to be the clobber element of it, but that’s gone. There are people who run about in designer gear who aren’t casuals.
The two of them go hand in hand in Europe, Ultras basically are the casuals outside of the stadium.
On the last point you made, the rule book doesn’t have to be followed. But the mentality has to be followed.
Stewards telling you to sit down? Imagine that in Europe? They’d get hit over the head with a pint of beer.