An SCL file is a chord library used by Sibelius, music notation software used to create scores. It contains one or more chord diagrams, which display the fingerings (finger placement in a musical score) for playing instruments such as guitar or trumpet. SCL files are useful for searching chord diagrams and sharing them with other users.

You most likely will only see the SCL file if you share a chord diagram library with another Sibelius user. Otherwise, the Sibelius program allows you to create, open, edit, and remove chord diagrams directly through the interface. To create a chord diagram library, select Create → Chord Diagram → New Chord Diagram Library.... You can add a chord to the library, select the chord and click "Add to Library." After you create a chord diagram library, Sibelius saves it in an SCL file and stores it in the "Chord Shapes" folder, which is located in the "Resources" folder in the main "Sibelius" program folder.