When we execute a command or call a program, the AS/400 MUST know where to find the command or program and the answer is library. A Library is a collection of objects. QSYS is the only library that contains other library. QSYS is the root library where the entire user defined/ SYSTEM defined library is created. System supplied libraries begin with the letter "Q" or "#".
Library list types:
System library: All IBM supplied library e.g. QSYS, QHLPSYS, QUSRSYS…
Product Library: Whenever Ibm product is USED it is added to the library automatically and is removed itself when the job completes.
Current Library: Current library is the working library i.e. all the work done by you is stored in current library.
User Library: Non-IBM supplied i.e. created by the user.

When we execute a command or call a program, the AS/400 must know where to find the command or program and the answer is library. A Library is a collection of objects. QSYS is the only library that contains other library. QSYS is the root library where the entire user defined/ system defined library is created. System supplied libraries begin with the letter "Q" or "#".
Library list types:
System library: All IBM supplied library e.g. QSYS, QHLPSYS, QUSRSYS…
Product Library: Whenever Ibm product is used it is added to the library automatically and is removed itself when the job completes.
Current Library: Current library is the working library i.e. all the work done by you is stored in current library.
User Library: Non-IBM supplied i.e. created by the user.