Frame installation depth

This is the thickness of the window profile

Greater depth = space for more chambers and seals, and more stable large sashes

The frame installation depth is the thickness of the window or door frame itself, specified in millimetres (mm), and it is a key factor in thermal insulation performance because the deeper it is, the thicker the insulating glass that can be used and the better it insulates, which is reflected in a lower heat transfer coefficient. ( Uf )

It directly affects the overall thermal efficiency of the window, expressed by the Uw coefficient (for the entire window) – link. The greater the profile depth, the better the insulation will be.