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Revit OpEd: Revit 2012 Materials and Property Sets
Creating an Appearance Library - WikiHelp
Also from the WikiHelp on this subject:
Each material can have two types of properties referred to as aspects: appearance and structural. You can store appearance and structural property sets in their own library. Appearance property libraries adhere to the Autodesk standard Protein format and have the following file extension: .adsklib.
Structural property libraries are written in an XML format and have the following file extension: .adstlib.
When a user appearance property set library is loaded, it is always available in the Materials dialog. Structural user libraries are available per project. They will not load unless you specifically load them into a project.
You can create new property set libraries in Revit or by creating .adsklib files with an external editing application.
Managing Property Set Libraries - WikiHelp