It specifically fits the Milwaukee 48228435 PACKOUT Compact Organiser.

This is the specific tool the insert is designed for: https://www.milwaukeetool.com.au/power-tools/nailers/staplers/M12BCST0.html

Specialised Storage for Staples:

To ensure your staples stay put during transit or the use of the carry handle, the insert has a custom designed rail system.
The staple storage area under the tool has overhangs designed in which prevent the staples from falling out.

These slots are designed so you can double up your staples. There is a spot closest to the Packout boxes hinge is where you load the staples from. Simply place it in that spot and slide them down along the storage channel.

Insert Use and Fit Note:

The M12 Cable Stapler is a uniquely 'long' tool thus can present a snug fit when placed into the compact organiser.
The main culprit for this tighter fit is the rubber padding present at the base of the tool next to the battery slot.
This design incorporates a reliable way to allow for storing the Cable Stapler in the compact organiser.

Follow the numbered guide in the gallery above for a visual demonstration in addition to the written breakdown below. It's an easy solution though:

Make sure the belt hook will be facing down into the insert
Insert the tool by placing the battery slot in first so it's resting in the bottom of the insert.
Lower the head of the tool down while keeping the battery end seated.
The tool is successfully stored! Simply do the reverse to take the tool out.

Some folks have shaved a couple millimetres off the rubber grip situated next to the battery slot to allow for a much simpler fit before now.
This insert allows you to store the tool without doing that.

Storage Channel Note:

You can fill the storage channels to full capacity with staples if you'd like. Just note, if the staples are filled past where the overhang section is closest to where the Packout boxes hinge is, the M12 Cable Stapler will place a small amount of pressure on those couple staple bundles when the lid is closed. This is only a minor factor but worth noting regardless. This was the trade off for being able to fit BOTH the Tool and then having storage underneath to maximise space.