Custom IDFs allow you to migrate COM APIs to .NET replacements. This is a powerful and fairly intuitive technique, but it is not as maintainable as Migration Rules files.
The gmStudio samples use custom IDFs files, but this approach will be phased out in favor of using Migration Rules files. See Custom COM Replacement.
gmSL scripts may be used to implement dynamic translation rules for various COM migrations in concert with custom IDFs and mig files. gmSL Scripts may also be used for special purpose analysis and transformation on their own.
Note we also offer a .NET API that has all of the power of gmSL and may be used to build custom translation engines as .NET executables. Learn more about gmsl here: gmSLIntroduction.