Many people with an existing, non-radiant slab would like to enjoy the benefits of radiant heat, but don't want to jackhammer their floor, haul away tons of concrete, and re-pour a new radiant slab. Can't say I blame them. The solution is a new suspended slab, of course, and one of the techniques for accomplishing this is illustrated in the photos below.
From here, the tubing from each circuit will either "home run" all the way back to the Zone Manifold, or (preferably) connect to a "slab manifold" or "site-built header" closer to the zone. In the latter case, two insulated 3/4" copper pipes, one for supply and one for return, will connect the "slab manifold" to the Zone Manifold.
The advantage to this "slab manifold" method is simplicity....especially for zones with many circuits. Without a manifold close to the zone, a pair of tubes from every circuit (supply and return) has to get back to the mechanical room. In a zone with six circuits, for example, that's twelve PEX lines coming and going from the mechanical room to the zone. There's no easy way to cleanly manage that kind of tubing cluster.