A fixture package is an archive file (with the ".sfixture" filename extension) that contains the boundary conditions describing the environment for testing a script. Its contents can be configured using the Fixture pane in SwitchScripter.
A fixture package is always stored alongside the script package (with the same filename but a different extension). The fixture is not part of the definition of a script and may be removed after the script has been tested.
A fixture package often contains references to test files. It just remembers the file paths, and never includes the test files themselves. The test files are externally maintained by the script writer/tester.