A performance shouldn't really be thought of as a file - it is the playable sounds of the montage (which could be single or multi part). A "virgin" montage will contain only preset performances which cannot be overwritten but can be stored as a new performance. This will then reside in the user area of the memory with each new performance added sequentially. You can also edit these user performances but in this instance you have the option to overwrite or store this as a new performance. All data in the user area remains intact on power off but it's good practice to save this externally on a USB stick. In this instance when you press Store you select User from the drop down Content Type list and then save this to a USB stick that you will have connected to your Montage.

You are then able to Load this user file back into the user area of the memory but be aware that all data currently in the user area will be overwritten (the montage will warn you of this).

A library file can be thought of as a specific variant on the user file in that it stores most of what a user file would (the reference manual gives the exact data stored for each) the difference being that when you load a library file it goes into 1 of the 8 library areas of the memory leaving the user area intact. All the performances stored in the libraries are accessible at the same time and can be edited though it's probably best if you want to maintain consistency that you import a performance from the library to the user area so that the associated waveforms (if there are any) become part of the user area - and hence part of the new user/library file when you save it.