When you upload, move, or copy a file to a document library, Windows SharePoint Services determines if a parser is associated with the document's file type. The parser extracts all the properties and matching property values from the document, and adds them to the property bag object. Windows SharePoint Services accesses the document property bag and determines which properties match the columns for the document. It then promotes those properties, or writes the document property value to the matching document library column.
Windows SharePoint Services can also invoke the parser to demote properties, or write a column value into the matching property in the document itself.Usefull Links: