Saving Multisession compilations is not absolutely necessary as Nero automatically saves all of this information to your CD when a Multisession compilation is burned.
Even so, just like ISO compilations, Multisession compilations can also be saved to your hard disk as a Nero document. The Multisession CD retains both the references to the reference track and a kind of 'fingerprint'. This fingerprint contains characteristic features of the CD and is used to identify the associated reference CD so that incorrectly loaded CDs can be ejected.