What is an extension and what is it for? Let's look at the not-so-distant past. Previously, each enterprise strictly adhered to the current GOSTs for its products: this also applied to building materials.
The regulations established the sizes of panel slabs, bricks, blocks, etc. Construction organizations also had standards for the thickness of plaster.
Based on the standards, woodworking companies produced standard doors and windows with constant size indicators. They fit precisely into the prepared openings, without requiring any finishing or additional adjustment.
Today, things are different. Most bahamas mobile database do not follow any generally accepted standards, but their own TU (technical specifications).
The consequence of this is that the same material (block or brick) from different manufacturers may differ in size.
It is unlikely that anyone will be able to name the exact thickness of the walls of a building now, so it is practically impossible to make all door frames and window openings standard. A fairly simple solution was found - door extensions.
Purpose
Now door frames are manufactured with the minimum acceptable widths, ensuring their proper strength. And by installing extensions, depending on the size of the walls, the missing width is made up.