Uke grabs one of tori's shoulders, more specifically the fabric of the jacket at the front of the shoulder.
It is in Uke's best interest to keep his elbow down while keeping his arms and shoulders flexible.
Kata-dori is not the goal of the attack, but only the first contact. Uke wants to use it to hold tori down or move him into a position where the attack can be continued,e.g. with a kick, strike or choke hold.
