Thank you.
Next question:
Does a module have to have the same version number as the installed trythond?
Example: Would the module m9s-account-invoice-discount 7.0.1 be compatible with trytond 7.6.9?
The version string of a module indicates the API compatibility to trytond version. You can try to use a module on a later trytond version, but if there are errors, the module will miss some adaptions necessary for the later trytond version resp. its dependencies (e.g. account_invoice).
The convention (at least for standard module) is to keep compatibility inside the same series (the 2 first number of the version).
But we can not ensure everyone who is publishing modules is following this convention. But at least they should correctly states the dependencies in the package setup.
Theoretically it is possible to create module that is compatible across multiple series but it is more complex than supporting a version for each series.
No it is not.
module version must start with 7.6
module 7.6.3 is compatible with trytond 7.6.9 and uncompatible with trytond 7.4.9
That’s quite misleading. It is like Cédric says: As long as the naming convention of standard is followed, a module is targeted at the series (first two numbers). That doesn’t automatically mean that it will not work with later series.
For example, in module account V7.6 requires.txt, I read “trytond<7.7,>=7.6”
I think installing this module on a trytond V7.0 would break it unless you edit this constraint. Am I wrong ? Do you mean that then it may work ? I never tried it.
You are right that an installation via unchanged setup.py could mix up versions, so you would have to edit that constraint. But you could run it from sources without impairing the rest of the dependency chain.
Disclaimer: I am talking about theoretical possibilities here, not something I would recommend to someone with little experience.