Yep, but if you look for tickets, for example, the Def Leppard+Motley Crue show, you'll see this:
PL1 FLOOR A 115,00€ + 15,50€ gastos
PL2 GENERAL STANDING (Grada General) 80,00€ + 10,50€ gastos
PL3 FLOOR B 65,00€ + 8,50€ gastos
There was a 25K attendance to this show and if you search pics of the venue, there's no "seats", only a bunch of rock seats with no numbers on them.
Literally there's a "first come, first served" with this kind of tickets.
There's no other venue in Madrid with this capacity, the closer one is Wizink Center (now Palacio de los deportes) (an indoor arena, 15K) or Civitas Stadium (home of Atlético de Madrid club, 65K)