IBSS

A stripped down version of iBoot, missing things such as interacting with the /|filesystem. Can be uploaded via DFU to bootstrap iBEC during a DFU Mode restore.

Use of the iBSS
The iBSS bootstraps the iBEC, which prepares and executes the Restore Ramdisk. in addition, it sends messages to iTunes on the restore to supervise the restore process. It also integrity checks the images uploaded, and on iOS5+ does the image responsible for APTicket, by uploading the nonce string to iTunes then checks for the match of the APTicket and nonce and the signatures on APTicket. On custom firmwares, the iBSS is patched out of every signature check, but on certain circumstances it still generates nonce. Check APTicket for further detail.

On jailbreak softwares like redsn0w and greenpois0n, the iBSS bootstraps iBEC and executes a payload. It is patched out of its signature checks, of course.