What constitutes a sellout in baseball?

Answer 1

Each club sets their sellout threshold, which is not necessarily the seating capacity of the stadium, and is not necessarily consistent between clubs. Clubs like the Phillies, Red Sox and Cubs, who all sell standing room tickets, may actually see attendance greater than their seating capacity. Other clubs may set their threshold at less than the seating capacity of the stadium and justify the difference as their accounting for comped tickets as only sold tickets count towards a sell-out.