I think I found it!
Actually I have used the name of the product which is SC5650BRP (this is for Europe)
Doesn't really matter - It's just a text string to log system information. You can use the TA#, or just answer "No" when prompted to update this information. Same for system S/N, asset tag, etc.
you can find the partnumber from the manual document that is shipped with the server.
Note: its not mandatory to enter this field. this is just to identify the servers if it goes wrong based on the SNMP mail.
You can name the servers as you wish like WS1, WS2.. as long you can refer the correct server.