Signature file used by Visual F#, a component of Microsoft's Visual Studio software development kit; contains information about the F# program element's public signatures, including types, namespaces, and components.

FSI files can be used to specify accessibility rules to program code within an .FS file. This technique is used to restrict access to other program code that executes outside of the controlled FS file. FSI files always have the same filename prefix as their associated FS file.