A library is a file composed of items that can be copied to and from snapshots. You can sort them, comment them, and copy them into other libraries. A library is stored on disk with a .tx8 extension. The file format is specific to the TX81Z Programmer, but you can export library items to regular sysex files en masse if you want. You can also import sysex files into a library. There is no limit to how many items you can store in a library, other than available memory.