The FSI file extension is primarily associated with the F# programming language, a functional-first, strongly typed, multi-paradigm language that targets the .NET platform. An FSI file serves as an interface file, which defines the public signature of an F# implementation file (typically with an .fs extension). It acts as a contract, explicitly declaring which types, values, and functions are exposed to other parts of a project or external consumers, while hiding internal implementation details. This separation of interface and implementation is a common practice in large-scale software development to improve compilation times, enforce encapsulation, and provide a clear API surface. When an FSI file is present, the F# compiler uses it to validate the corresponding FS file, ensuring that the implementation adheres strictly to the defined signatures. This is particularly useful in library development where maintaining a stable public API is critical for backward compatibility and modularity.