Tone is a muscular engagement in the body. It is characterized with only a single muscle of a matching pair in the body being engaged. This allows connection to the partner who engages a single muscle as well to counteract creating association and balance. Tension is a similar principle but includes both paired muscles being engaged at least partially in a single person.
We aim for using tone in partner dance over tension due to the fact that tension uses additional muscular engagement and creates mixed signals/less data for proprioception. This does not mean Tension has no uses see Tension.