CMake/Help/prop_tgt/LINK_LIBRARIES_INDIRECTION.txt

.. note::
  A call to :command:`target_link_libraries(<target> ...)` may update this
  property on ``<target>``.  If ``<target>`` was not created in the same
  directory as the call then :command:`target_link_libraries` will add a
  suffix of the form ``::@<directory-id>`` to each entry, where the
  ``::@`` is a separator and the ``<directory-id>`` is unspecified.
  This tells the generators that the named libraries must be looked up in
  the scope of the caller rather than in the scope in which the
  ``<target>`` was created.  Valid directory ids are stripped on export
  by the :command:`install(EXPORT)` and :command:`export` commands.