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The Exclusive Chip ID or ECID, is an identifier unique to every unit.

The Img3 Tag

With the iPhone 3GS, the ECID tag with your ECID is put in the signature checked section of every firmware file, and sent to Apple's servers for re-signing.

The implication of this is Apple could disallow downgrades (even via DFU) on newer devices when a new firmware version is released.