2017-05-27 14:15 GMT+02:00 Thierry Goubier <thierry.goubier@gmail.com>:
Will:

#MyPackage asPackage definedClasses do: [:each | each tagWith: #MyPackage]

put the classes in the #MyPackage or in the #MyPackage-MyPackage system category?

#tagWith: not touches package of class. It only adds receiver into package tag.
I not understand the question about system category because I don't know what it is exactly in current system.

Now in class definition we have "package: " part. And it will be modified with new tag-suffix. It will become "package: 'ActualPackageName-TagName'"
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Will that API stay the same when we will be able to put a class in multiple categories (or a method in multiple protocols)?

Yes but #tagWith: will add extra tag into class/method. It will not remove existing tags.
But in Pharo6 it is only new API. Multiple tags are not supported and #tagWith: removes existing tag.