Uke grabs one of tori's wrists with one hand.
It is in Uke's best interest to make it a good and secure grip while keeping his arms and shoulders flexible ("sticky grip").
Katate-dori is not the end 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 strangling grip.
If only Katate-dori is requested, it always refers to the Gyaku-hanmi position.