1) Fans with upper deck ("Tier") tickets can walk around on the main aisle on the Field Level. The individual aisles to the Field Boxes are restricted to ticket holders, and it is those seats that are chained off, as shown in this photo:
Originally Posted by NYFan1stYankFan2nd
(Photo taken June 21, 2008. © Gary Dunaier)
The main aisle is approximately twenty rows away from the field.
2) Fans with Bleachers tickets are restricted to the Bleachers, unfortunately; they are not allowed access to any other part of the Stadium.
Archie Bunker: All I can tell you, Edith, is I'm surprised at you! Dragging me off to a moving picture like that! It was absolutely disgusting!
Edith Bunker: Well, I'm sorry, Archie, how was I to know? I thought it was a religious picture, "Cardinal Knowledge!"