We fix this aluminium track to provide a level “starting-point” for your installation, and build the insulation panels upwards from here. The track also protects the system by preventing attacks from rodents. It’s always kept above path-level so that any water collecting on the path can’t work up into the system, and the space can be cleaned (e.g. in autumn, when blown leaves might gather there).
You can also see a plastic bead is clipped in place – this gives a slight “bell-cast” effect.