The M visa for business
M is the commercial visa: trade fairs, negotiations, site visits, signing. It is not a work visa, and the line between the two is where applications go wrong.
If you will be paid by a Chinese entity or employed in China, the visa you need is Z, and applying for M instead is not a shortcut that survives contact with the border.
The invitation
The document from China does the work here. It comes from the company or organisation you are dealing with and it has to say who you are, what you will be doing, when, and who is covering the cost. The consulates publish a note on what an invitation must contain, and files are returned for missing exactly those points.
Reviewers who apply for M visas regularly describe the process as smooth once the file is approved online. The friction is almost always in the invitation, not at the counter.
M and Z, and the line between them
The distinction is about payment and employment rather than about activity. Attending a trade fair, negotiating a contract, inspecting a supplier and signing an agreement are all M.

Being employed by a Chinese entity, or being paid from China for work done there, is Z, and Z requires a work permit notification issued in China before the consulate can do anything at all.
Applying for M when the trip is really Z does not usually fail at the consulate. It fails later, when a residence permit is refused or an employer's paperwork does not match the visa in the passport.

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