There is currently some discussion about aspects of the new avatar bones being used in odd ways. The third party viewers in some cases have adjusted some things in the upload code to allow use of bones and joints in ways not completely support by the Lab.

As the discussion goes, those aspects may be blocked at the server soon. That would mean things like weighting mesh to the attachment points would no longer be allowed. We do not know whether the Lab will or won’t decide to block such use. So, don’t panic. But, if you have a good use case for your uses, get it into a Feature Request as soon at the JIRA’s new Feature Request channel opens up.
I am not done with my testing of the new avatar yet. But, so far there is no model avatar I can find that is in use in Blender that can be imported to SL and behave as the SL avatar does. The supposed exact same avatar is not an exact copy nor do the sliders affect it in the same ways. It is close, but not the same.

