More information here https://www.fusion-bags.com/collections/fuse-on-bags/products/urban-small-backpack
Just "fuse-on"
Made from 10mm (0.4ins) high-density foam padding and non PVC backed water resistant rip-stop material, the Urban Small "Fuse-on" bag can be attached to the headstock area of the Urban Guitar bag, or to the front of any smaller sized Urban Brass & Woodwind bags.
This bag is ideal for brass & woodwind musicians who need extra space to carry mutes, as the padding helps keep everything well protected. The same applies to guitarists who need additional space to carry leads, microphones or foot pedals to a gig.