When stocks are created, the information about the stock is included in the part file. For portability, this definition is used in the NX session whenever the part is loaded.
As stocks are standard, its expected that changes to the definition will be rare, but not never. There is now a way to update stocks to the current data using Refresh Stock Definition. For bulk changes this is also available using NX/open.
Update stocks created with incomplete or out-of-date definitions easily
Update stocks with new attributes to enable new or previously unused functionality