Module is shared namespace of APIs. The module file is created by the compiler.

When you build your project that has pre-compiled library. You had to match Swift compiler version with current and library.

Because build system using .swiftmodule for matching modules API with code current compiling. If the version of .swiftmodule are different, compiler cannot understand another version of modules API.

But in Swift 5.1, provide .swiftinterface for provide compatible across different version of Swift compiler.

That means you can compile binary even library built with different version of Swift compiler.

Attribute for better performance of Module Stability

@inlinable It can cross deliver function also body to the another module.

@usableFromInline If inlinable function has internal property, It can open to another module for using inlinable.

CAUTION: You should not change behavior of inlinable function because User of module steel using older version of inlinable function.

@frozen It promises enum cases, struct property, will not added.

Reference https://developer.apple.com/videos/play/wwdc2019/416/