The syslog-ng Store Box (SSB) firmware is separated into two parts: a boot and a core firmware.
The boot firmware boots up SSB, provides the high availability support, and starts the core firmware.
The core firmware handles everything else: provides the web interface, receives and processes log messages and so on.
When you upload a new .ISO file using the SSB web interface, it updates both firmware. For details, see Upgrading SSB.