Create an ad-hoc Symbol - P&ID

Question
In AutoCAD P&ID, if I need to create an ad-hoc symbol I can use the Convert to P&ID Object command but I can only choose certain classes of symbol. Why is this?

Answer
The Convert to P&ID Object command allows you to select classes of symbol that have a GraphicalStyleName property defined. The command does not allow you to create a new type of symbol - it is designed to allow you to create a new ad-hoc representation of an existing type of symbol.