UCS backup that contains encrypted passphrases

Could you advise what kind of passphrases it is referring to in

"If you are restoring a backup that contains encrypted passphrases after reinstalling the operating system, replacing a failed system with a new system, or otherwise moving an existing configuration to a new system, the encrypted passphrases for objects used in the configuration cannot be decrypted. An error message similar to the following example appears:0107102b:3: Master Key decrypt failure - decrypt failure - final"

This is from: sol13132: Backing up and restoring BIG-IP configuration files (11.x)

I can only assume that those encrypted passphrases are located within the configuration, right ?. If so, where are those passphrases located in the LTM ?.

Thank-you. Edouard.