No parameters for this function.
This function is called "InterfaceFile" because it might typically be useful when using the so-called Separation Model of FileMaker development. The function is nothing but a text string. The point of it is in this technique:
In some scripts or calculations we need to reference a file name as a text string (e.g. "Main Menu"). When the file in question is the same as the calc or script, we can avoid hard-coded strings with the Get(FileName) function. If it's a different file, however, we must supply a hard-coded name.
The problem is, if that file name ever changes, we must manually change our references one by one. This is tedious and error-prone. Instead, wherever a hard-coded string is required, reference this custom function instead. That way if you ever need to change the file name (perhaps just for testing purposes), you can do it easily by simply changing the custom function.