Remember there is something like 28 ap's for the step format.

I don't think that the extension denotes any of these. It is just an application preference.

Upon import, you usually find a selection of (step,stp) to "see" both file type extensions. Some apps give you 2 exports just to be able to "write" the extension of choice, but it would be no different than renaming the extension yourself.

I think it's up to the particular translator to solve the contents of the file.