To regulate over crowding Disney does a few things:
Level 1: Normal entry and re-entry to the park
Level 2: The park stops selling admission tickets (about 60,000 guests) Pass Holders and pre purchased tickets are still allowed entry.
Level 3: Admission only to hotel guest and readmission, with hand stamp for Pass and ticket holders. (about 75,000)
Level 4: Close the park no one enters the park.
More...