Rogers PCB Dielectric Constant Design (DK) should be used for simulation and design.

• The process DK is tested by IPC defined test method.  Tested coupons are raw dielectric material - this process of testing is used to monitor the dielectric performance, to check if it meets the manufacturing test limits.  That is why it is named Dielectric Constant Process DK.
• The design DK is tested by Rogers defined test method-microstrip differential phase length method.  This method uses microstrip transmission line which is very common in customer's real designs.  The design Dk generated from this method is the same as circuit's behavior.  So Rogers named it as design Dk.
• The design Dk nomally has a little higher Dk than process Dk.  Process Dk is real dielectric Dk, but because of copper roughness effect, which slows down signal wave propagation, at the end, design Dk has a little higher value than process Dk.
• Difference copper types have different impacts on design Dk.  Rougher copper has more effects on design Dk, that means, design Dk will be higher on rough copper than smooth copper.
• The smoother the laminate's copper is at manufacture, the closer the process Dk will be to design Dk.