Dear,
In operating cases with seismic, there will be thermal displacement (D1) and imposed seismic displacement (for example D4 for U1).
D4 works as D1. D1 imposes a thermal displacement to your pipe, like a nozzle connection when you are not modelling the equipment.
D4 imposes a seismic displacement to your pipe. For example, you are guiding your pipe from a T-Post support which is connected to the ground. That support, because of the seism, will move. You can model the structure and connect (CNode) your pipe, or directly, impose that displacement with D4.
Hope that helped. Regards.