First of all User Defined Functions as specified within the Formulas properties of your app. Just like Named Formulas. In the above example we can see that the structure is fairly similar.

User Defined functions are data structures in Power Apps that specify the structure of a record. They can typically be used to convert an untyped record into a typed record.

You can create custom Excel functions using VBA matching using your criteria. Create custom Excel functions using Macros in Excel & use user-defined functions in Excel.