The effect of the ref node of the s2p data item in ADS

Hi everyone, I encountered a problem in my simulation using ADS. Usually I always connect the ref node of the 2 port s-parameter item to ground, and it seems working well.
Yesterday, I used design guide in ADS to generate a interdigital capacitor,and simulate it in Momentum as a two port network.The s parameter got from the simulation was imported to a 2 port s-parameter data item.Problem arised that different results were obtained when I let the ref node of the s-parameter item unconnected.
What's the effect of the ref node,help me! Thanks!

The ref node represents the ground reference of your circuit.
If it is not connected to the ground,it means the ground of your circuit is floating.