It specifically fits the Milwaukee 48228435 PACKOUT Compact Organiser.

The specific Dremel that this is designed for is model 8260:
https://www.dremel.com/au/en/p/dremel-8260-f0138260na

Features - Super Flexible Design

Double Layered Storage - Two lift out trays reveal even more storage underneath the tool itself and the main bit storage area.
Battery Storage - Room to store your tool with a battery installed plus room for an extra in the included storage tub.
No Loose Dremel Bits - Designed to follow the contours of the PACKOUT lid when closed. You can rest assured you won't have your Dremel bits jumping over dividers whilst in transport or carrying your PACKOUT.
Quick Select - Dedicated storage slots included near the head of the tool for storing your most frequent bits for easy selection.