Er, how 'bout "according to the a/c mass & balance table + trim sheet"?
The free seating consept isn't really "free". The Companies know that people will tend to spread fairly evenly around the a/c by themselves - that's a given in Western culture - we wanna be alone. The cabin crew must nevertheless check that they have a minimum of x number of pax in certain sections. If they don't they'll have to move 'em.