Certain types of the object (database file, storage areas and executable program objects) can be compiled, copied, and stored into/from many different libraries concurrently, with the Library List hierarchy determining which instance of the object to use during execution of any application that utilizes that object name. There are commands to help move objects around, and objects may be qualified at compile time to point to specific other objects statically at runtime, in order to circumvent the Library List.
Other types of objects, designated as "system" objects cannot be replicated. Libraries are a "system" object, and therefore only one instance of any given Library "name" is possible. They're made to appear as if they're stored in the QSYS library. Other "system" objects include user profiles (*USRPRF), configuration objects (*LIND, *CTLD, & DEVD), etc.
Most homegrown applications have between one and three libraries. Some have program, source, and file objects together in one library. With shops that have a huge amount of data and a small backup window they usually split the database files into a separate data library and keep the source and programs in another library. In companies with a separate development systems they tend to split it further by placing the source into its own library.
Generally speaking all libraries created by IBM for use by the operating system begin with the letter 'Q'.
IBM Standard Libraries:
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