Shsh - Blobs

Apple checks if that version is still "open." If it is, they send back an SHSH blob.

Without this digital signature, your device will refuse to boot or install the operating system. Why Do They Matter? shsh blobs

You cannot save blobs for a version of iOS that Apple is no longer signing. You must be proactive. Apple checks if that version is still "open

Your device's bootloader checks this blob. If it matches, the installation proceeds. How to Save SHSH Blobs You cannot save blobs for a version of

An (Signature HaSH) is a unique digital signature that Apple uses to verify the firmware version you are trying to install on your device. Every time you restore or update your iPhone via iTunes or Finder, the software sends a request to Apple’s servers. Apple then "signs" this request with a blob specific to your device's unique ID (ECID) and the specific iOS version.