A User Bank consists of as many as 640 Performances
As many as 256 User Arpeggios
As many as 2,048 Waveforms
As many as 256 Motion Sequences
As many as 2,048 Live Set slots
As many as 32 Curves
As many as 8 MicroTunings

Think of a Library as the data contained in the User Bank transferred to a Read Only Memory location on the instrument’s Flash.
The difference between USER memory and LIBRARY, is the Library was once in User memory (where you can make changes to the data) - once it is installed on Flash it cannot be overwritten by simply loading a new file.

There are eight Libraries...
5,760 Performances....
5,120 in Libraries + 640 in the current User

When you load a file with “Content Type = User”, you overwrite the Performances, Arpeggios, Waveforms, Motion Seq, Live Sets, Curves and MicroTunings that are in User memory areas with a new set of Performances, Arp, Waves, Motion Seq, Live Sets, Curves and tunings.

Memory Rule: when you have filled the 1.8GB of Flash the memory is full. Waveform data (Sampled audio and the parameters that ‘map and catalog’ them) occupies the lion’s share of space... a set of Part parameters might be 2KB of Data, the Waveform and Samples might be 400MB or more.
Things like FM-X sounds hardly use any memory by comparison to any other.

Thanks BM - but how do I find out how many Pfs are on a Library file? For example, I have 16 Libraries, all with varying numbers of Pfs on them. I want to move things about or add to some of them. My plan is to find out how many Pfs are on one, then 'Library Import' to Montage and add it to the User files already there, then re-save all as a new Library - then delete the original Library. To do that I need to know how many Pfs are on the Library to start with, a check before I load the Library into Montage, or import it if already on the Montage.

I know I could load the Library into Montage and then 'Library Import' to the User Bank, but that's a very long-winded way to perhaps find out there's not enough room left on that Library to add what I want. So I wonder if there's a convenient way to find out like the User Bank has.

You go to Utility > Contents > Library Import, and you select the library.
You count number of pages times 12
.

@+

So I wonder if there's a convenient way to find out like the User Bank has.

If you have gone through the very steps you outline, the answer would appear directly in front of you. Not only the number of items in what you are getting ready to import but the number of available locations currently in User.

Actually the MONTAGE will tell you exactly how many Performances are in a Library.

Press [UTILITY]
Touch “Contents” > “Library Import”
Your installed libraries appear as boxes.
Touch the box of the Library you wish to view, to open that listing
Touch the box labeled “Select All”
A checkmark appears next to each ‘selected’ Performance name (thus the number of Performances in this Library);
the *count* appears in the upper right corner

The number that appears on the left is the number of “selected” Performances in the Library you chose...
The number on the right is the number of empty locations currently in your User Bank
Touch “Unselect all” to take no action.

AH! Thank you BM! That's it in a nutshell. I didn't go far enough - hesitated to 'Select All' in case it started immediately - tells me both the number of Pfs in that Library AND the number in the User Bank (640 less the RH number). BUT - I still have to 'Load' from the USB? No way to see how many Pfs are in the Libraries on the USB? That's mostly what I want to do, before loading, just to check that I can get the additions on. Saves the long wait to load and then delete ... if it's not possible, it's on my Wish List for the next update ...

Thank you Tino - I hadn't gone far enough!¬ I would still like to do the same on Libraries on the USB though. That's the more frequent - saves having to load it into the Montage and then having to delete it again if not suitable.