Le 2012-11-08 18:48, till a écrit :

[...SNIP...]

A plugin is essentially powered by the `composer.json` file.

And this is how composer works (in a nutshell):

 * it takes the URL of the repository (git, svn, hg)
 * it scans all tags and branches (e.g. trunk/master) for `composer.json` files
 * it will use all tags/branches where it finds a `composer.json` file

E.g. (for SVN):

 trunk/composer.json
 branches/super-feature/composer.json
 tags/1.0.0/composer.json
 tags/0.9.0/<no composer.json>

The plugin repository would detect:

 * dev-trunk
 * dev-super-feature
 * 1.0.0

[...SNIP...]

Ok, thanks Till for all your explanations. This is far clearer now, at least for me.
Last question about this particular piece : 
what happens if the trunk/composer.json contains the same version than, say, tags/1.0.0/composer.json ?
should I ensure that in the whole tree no composer.json file has the same version in it than an other composer.json file ?

regards,

S.B.