There is always the possibility of visual separation, in that case ATC hands the responsibility of separation to the crew (of the succeeding) aircraft and may only give them a minimum limit (do not overtake preceding aircraft in FRA for example).
And then there are some airports where minimum separation is reduced on final down to 2 NM between same category. And of course others (one is famous for it in spain, where else) where they apply wake separation behind 737-800s with heavy category values
