From the designer:

This is a custom insert that I have designed to fit your Milwaukee Packout to store your M12 23 Gauge Pin Nailer and accompanying kit. It specifically fits the Milwaukee 48228435 PACKOUT Compact Organiser.

The insert consists of 2 parts, the first is the insert for the Pin Nailer itself. I've part of the insert to have an empty space underneath the tool where you can store even more extra nail packs. The second part is an organizer insert for your extra boxes of nails and a spare battery.