Hello

It's more like "professional category" and it goes far beyond "collective agreement" in that there may not even be an employer, as with freelancers or sole traders for instance.

There may also be "legislation" covering such categories, for example qualifications and tax returns, which go beyond the idea of "collective agreement"

See a French tax return form for landlords for instance.

There's a reference on a PDF file to a minimum conventional salary.
https://www.usoenc.nc/wp-content/uploads/2023/04/Salaire-Con...
At a business convention (a meeting or a conference) a group of people or interested parties held a discussion to reach an agreement on minimum salaries for employees.
(A (business) convention is an event where a group of people interested in one topic gather at a specific place and time to discuss a said topic or make a decision.)
https://dtenc.gouv.nc/reglementation-en-ligne/conventions-ac...

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Note added at 4 days (2023-11-23 17:48:38 GMT) Post-grading
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A conventional wage or salary is one which is accepted as a standard basic rate of pay.
https://www.gov.uk/national-minimum-wage-rates
Here are some definitions of conventional:
"A conventional item is based on, or in accordance with general agreement. It conforms to established practice or accepted standards or traditions.
In law a conventional item is based on consent, agreement or contract.
It's established by general agreement or accepted usage."
https://thefreedictionary.com/conventional