There are criticisms about the way items are tagged in "Web 2.0" application. Yes, that's true free tagging is sometimes confusing and lead to issues to clearly identify and tag an object. There are sometimes confusion around the tagged object, do we tag the reference of an object (e.g. a LibraryThing link to a book) or really the object (e.g. the book itself) ? The criticisms can be justified but we must not forget that we are coming from the hierarchical and fixed classification. But free tagging works and open the world of "classification" 1 to anyone. It's easy to find content in social web services (e.g. flickr) using the tags given by people without predefined fixed classification. That's nice and it works better than any imposed classification.
But sometimes in free tagging, you want to define extra information about a tag or define a specific value to a tag. The machine tags are there for providing extra information about a tag and better define the scope of the tagged object/reference. For a good introduction, there is a nice explanation about the machine tags implemented in flickr. In that scope, the current practices for tagging the license to an object/work/reference are variable and introduce confusion (Now I hear the criticisms coming from the backstage about free tagging ;-). In that scope, I'm trying to define a specific license namespace to clearly define the license of a tagged object. The idea is to use a well-defined predicate in the license namespace to avoid license confusion.
where gpl-3 predicate clearly defines the following :
GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version (ref:http://www.gnu.org/licenses/gpl-3.0.html).
1. Yes, I should not use the term classification for free tagging. I'll burn in hell for such statement. A lot of work meeting looks like Hell to me. So I'm not afraid…