The essential difference between a docx file and a docm file is that a docx file cannot contain macros when saved. You can, however run macros against them; you can even add a macro to a docx file if you don't mind losing it upon closing the file.

Your addin should be saved as a dotm file (ie a word template), thereby making its macros available to all documents based on that template. Indeed, if the 'special toolbar' is appearing in the docx files, I suspect that's exactly what has happened.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]