When you bring up the SAS System, the libref SASUSER is automatically associated with a permanent SAS data library. 'Permanent' means that the SAS data library (and all files in it) will be available for use in later SAS sessions.
The SASUSER library is a convenient place for storing SAS files (data sets, catalogs, and so forth) that you want to be able to use in future SAS sessions. You have write access to this data library, and it is unique to you. Other SAS users have their own SASUSER libraries.
Note that entering or exiting SAS/LAB software does not affect the SASUSER library. The SASUSER library is available before, during, and after your work with SAS/LAB software.
SAS/LAB software makes extensive use of your SASUSER library. It saves data sets and other items to the SASUSER library so that information used by SAS/LAB software is not lost between sessions.