[NTG-context] Proper setting of font features for protrusion/expansion

Dear All, e.g. in the last example at http://wiki.contextgarden.net/Protrusion there is the following definition: \definefontfeature[default][default][expansion=quality,protrusion=quality] I understand it overrides default settings and adds these two features. In my code I use the following shortcut found somewhere: \definefontfamily[palatino][rm][Palatino Linotype][features={default, quality}] Both features works, but I am not sure if I am getting the best quality. Is there any description what that '[features={default, quality}]' option really does? Thanks, Jan