From what I gathered it is a metadata binary file that is created for a library and/or a cell, etc. that provides additional information about that library or object it's created for. So in the instance of a prop.xx file for a library, it may contain the attached techLibName. I believe this file is created to hold "properties", so it is basically created when you create properties to a library or cell... The manual calls it a "property bag" file I believe, so it seems to be a bag that holds all the properties for an object. Is that about right? Corrections and additions are appreciated. Thanks.