Tail up on a cat simply means they anticipate the interaction to be friendly. I remember taking my office cat (Boss) to a neighboring furniture showroom. It was his first time in the space and he was very curious but cautious. A cautious tail is held down, closer to the floor. He'd walk through the furniture sniffing and inspecting with a down tail and when he got within four feet of a person the tail shot straight up in the air! One he moved past the person the tail would go back down.
A vertical tail precedes both sitting down with another cat and rubbing on another cat. Pet cats approach a tail-raised silhouette of a cat fasther than one with its tail down, confirming that the vertical tail signals an intention to interact amicably (Bradshaw and Cameron-Beaumont, 2000).