From ImageStream Router Documentation
Add-on packages greatly enhance the capabilities of routers running ImageStream Linux by allowing the installation of software on a hard disk or flash drive. ImageStream introduced add-on packages with the 4.2.12 and 4.4.0 distributions. Routers with a minimum 128 MB flash module can install some add-on packages. ImageStream also offers add-on hard drives or solid state disks (SSDs) for most router models.
Add-on packages depend on the add-on hard drive service. This service provides the system with the disk space required for read-only package software and read-write data storage. Setup - Setting up the add-on hard drive service