Most partitions are framed with studs and tracks only, and everything else is fixed to the surface. The QU38 rail is different: a 38 × 12 mm, 0.8 mm section that passes through a square hole punched in the vertical stud and is limited by it. Each crossing is a mechanical lock rather than a contact.
The structural effect is that the studs stop acting as individual columns and start acting as a braced grid. Lateral movement is restrained at every crossing, so a tall wall does not bow between floors, and a wall carrying heavy fittings does not rotate its studs. On a 122 mm wall this is what allows door openings and heavy hung equipment without adding a separate frame.
The service effect is just as useful. The rail is pre-punched so cables and small pipes can pass through it, which gives the services a defined horizontal route inside the cavity. A route that is designed is a route that does not get cut afterwards — and cutting the frame is how acoustic walls lose their performance.
In practice the rail is set at the height where the wall needs bracing and where the services run, typically one or two courses depending on wall height. It installs with the studs, before the boards go on, so there is no going back to add it later. That is the point: bracing and service routing belong in the frame, not in the finish.