Does docx, xlsx, pptx format file meets the standards CNS15251?

The first question is, what is ODF-CNS15251?
When I search for it most of the hits are in pictographs.

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The only one readable was this one

https://blog.documentfoundation.org/blog/2019/08/15/libreoffice-asia-conference-report-part-2/

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It implies that the Word formats are not compliant with the standard. But given the source, you have to take it with a grain of salt.

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I would think that the standard itself would identify which file formats are compliant.  What does the standard say about MS Office?

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The new X-file formats are XML based, wrapped in a ZIP file format that is renamed for the appropriate application. So they are at least in the ball park.

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I found this link

http://www.pers.nptu.edu.tw/ezfiles/17/1017/attach/89/pta_79380_3284967_42220.pdf

and used google translate. It includes instructions on how to convert Office x-file formats to ODF formats. Basically, each of the applications has a SAVE AS ODF file format.

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So, bottom line, unofficially, apparently Office x-file formats are not compliant, but Office programs can save in the ODF file formats.

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About the standards, you can search 15251 here: https://www.cnsonline.com.tw/?locale=en_US

Seems the answer is no, so I have to refuse users upload Office x-file, although it's a ODF file but unfortunately it does not meets the standards.

Thanks.

I searched that link, but all of the results are paid. So, no, I can't look at them, you will have to.

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Yes, you have to tell users to not save as native Office x-file formats. Teach them to do SAVE AS OD* file format, ie in Excel:

Actually, there is an option you can set in the office programs to default to the correct file format:

When I was capturing this image I noticed the "Strict Open XML" file format. You will have to research to find out if this meets the of your standard, or if you MUST use the OD* formats.