From Halopedia, the Halo wiki
The Forge Objects lists serve to detail all objects available in Forge mode in Halo 3, Halo: Reach, Halo 4, Halo 2: Anniversary and Halo 5: Guardians, including prices, map availability, and the individual prices per map. Note that the price for an item can change if the player modifies certain properties of it.
Viewing an item
All Forge Objects are represented in the following format.
In the table, the "Cost" cells refer to how many Forge credits an item uses up. "Max" cells refer to how many of the item can be placed total. Greyed cells in the table mean that the item can't be placed on the corresponding map. In the example entris above, the greyed cells mean that the item cannot be placed in Citadel or Spire.