PCB Part - Ref Des

Hello, a quick question - when creating a part and then assigning a Ref Des name to the -part I am using the default
"REF"   (Silkscreen_Top) Ref Des button and the text on the Silkscreen_Top is "REF"    

What are you using for the default Ref Des  Text?   When I build the schematic symbol I will assign a "U" to this part.  How does it know that this U1 (for example)  will be assigned to REF? when I package it for layout?

Thank you