What identifier is meant in the Bitlocker recovery key document?
The identifier of the drive is generated when the drive is encrypted. This allows you, the end user, to identify which recovery key goes to which encrypted.
The reason the message says to "compare the start of the following identifier with the identifier value displayed on your PC." is because of how Bitlocker Recovery itself works.
Notice the Recovery Key Identifier part of the screenshot.
One would normally save the recovery key with the identifier in the filename like so.
How to Unlock a Drive using BitLocker Recovery in Windows 8 and 8.1