It's the BFA updated collections system. Each collections has an armature linked to a specific combination of race_sex. This armature permits to link the armor geosets of the 3D parts to the player models through weightpainting.
Old system, 7.0.X to 7.3.5 was only one weight on each mesh which were independant of each other and not the same as the other armatures for the same armor. All combinations or race_sexe for the same 3D armor had a different model, armature and weighting.
In BFA it has been updated, still the same way of processing but the armature system changed like the weighting. You can now as you see, weight paint whole pieces of mesh to be mapped to legs and chests. Which was not really possible in Legion. Still Legion handles this perfectly even if it's BFA.