A library that allows you to split your SCA library into pieces.
    Each piece is called a physical library. SCA works the same way
    whether your virtual library has only one physical library or
    several physical libraries.

    Some of the reasons for using more than one physical library are
    as follows:

    o  Physical libraries can be placed on more than one disk, if
       there is not enough room on a single disk.

    o  Analysis date files can be loaded into more than one physical
       library at the same time, to make LOAD run faster.

    o  A small personal physical library can be used to keep track of
       your personal changes without affecting other users.

    o  A separate physical library can be used for each major
       component in your system.

    A single virtual library is a list of one or more physical
    libraries. The order is important. A module in one physical
    library hides the same module in physical libraries later on in
    the list. This list of physical libraries is called a library
    list.