Board joints need something behind them. On a stud wall, a joint that falls between two studs has nothing to fix to, so the joint is either moved to a stud — which wastes board — or backed with a rail. The T40 rail is 40 × 7 mm, 0.7 mm thick, formed as a T rather than a flat strip, which keeps it straight over a 3 m span.
Forming the T instead of using a flat strip is a stiffness decision. A flat 0.7 mm strip sags between supports and twists when it is screwed; a formed T resists both, so the board joint stays on a straight line and the skim coat does not have to hide a wave.
The rail lengths are set to the stud spacing as a module, so the cut list follows the layout rather than the other way round. On a large floor plate this is what makes prefabrication possible: boards and rails can be cut to a schedule, delivered in the order they are needed, and installed without a cutting station on every level.
It is a small part doing an unglamorous job, and it is the one that most often gets substituted on site by “a bit of stud”. That substitution is usually visible afterwards: a joint that runs 2 mm off line over the length of a room, under a painted finish, in raking light. The rail costs less than the filler and the repaint.