Which units are placed in the box and when?
Units are placed there by turn event instructions. P22 and P23 of the rulebook.
These are two Divisions and the Monty marker:
7-13: Tobruk Reaction Box Activated: This box holds
the Allied units that become available as reinforcements
the Game Turn following the Axis capture of Tobruk. If
Tobruk has already been captured, these units are instead
received as normal reinforcements (or simply stay in play).
7a: ANZACs: During the Organization Phase, the Allied
player must permanently remove one Australian Infantry
unit, and place one other Australian Infantry unit in the
Tobruk Reaction box on the Player Aid Mat.
10a: Pacific Campaign: During the Organization Phase,
the Allied player must permanently remove one British,
one Indian Infantry and the 4th Tank unit; and put the
New Zealand Infantry unit in the Tobruk Reaction box.
12: Montgomery: The Monty marker is immediately
placed in the Tobruk Reaction box.