On the Mono compatibility page I see .net, .net subset, web and micro. When I open player settings for iOS or Android I can choise between .net and .net subset but not micro. So my question is - where is that “micro” used?
Micro is the stripped down mscorlib that is selectable from the Stripping Level (Pro feature) settings on the Build Player tab. See:
http://docs.unity3d.com/Documentation/Components/class-PlayerSettings40.html
and