I'm guessing that the symbols that come with AutoCAD P&ID have predefined attributes.
I suppose if you have equipment or valve blocks that were created using plain AutoCAD they too could be used. Get all 5,000 FREE CAD symbols and sample plans when you order CAD Pro today! Over 5,000 FREE CAD Symbols! You get all of the following categories free in the CAD Pro Software symbols collection, to meet all your drawing needs! Add power to your CAD Pro Software with a full selection of professionally drawn symbols and sample plans.