From Team Fortress Wiki

“

— The Scout on conflicting equip regions 'Kay, this does not look good here, um... ”

The equip region is an item attribute in the client schema used to identify the region where an item is attached on a character model. The equip region attribute (denoted by the value name equip_region ) is given to all Cosmetic items and some weapons, and its primary purpose is to prevent items in the same region from being equipped simultaneously. Equip regions were introduced to avoid visual clipping errors upon the addition of two usable 'misc' slots. This system was retained when the existing hat and miscellaneous slots were merged into three 'cosmetic' slots as a result of the December 20, 2013 Patch.

There are 68 equip regions in total. The equip regions of cosmetic items and weapons vary, but the vast majority of items that were classed as hats before the cosmetic overhaul have a base equip region of hat or whole_head .

Region conflicts

Equipping two items that have equip regions in both below columns, or two items that have the same region, is not allowed by the game.

Equip region Conflicts Glasses Face, Lenses Whole Head Hat, Face, Glasses

List of equip regions

*Tournament Medals are condensed to a single link, and items with the "Hat" equip region are omitted for brevity.

Bugs

It is possible to equip two items with conflicting equip regions simultaneously in these conditions: If one item equipped in the slot is removed from the Backpack, or the slot is otherwise unused and vacant, and the item preferred is equipped in the slot in another loadout. If the connection between the user and the Steam's servers is slow. Using the quickswitch key function, which does not check region conflicts when equipping items. If the user can put the two conflicting cosmetics on fast enough, then closing the loadout menu before it removes the conflicts.

The equip regions will not work on an item that has multiple equip regions, if it uses equip_region instead of equip_regions in the item schema.

instead of in the item schema. If an item has no equip region, it is possible to equip multiple versions of the item at once.

Items that do not allow use of certain cosmetics and weapons because of conflicting regions (for example, Thermal Thruster and any Pyro backpack cosmetic.) can still be used while the conflicting item is equipped.[1]

References