How the diagonal continues
A one-shoulder neckline may use a broad band of fabric or a narrow line across the bodice. The line can continue toward the waist, repeat in draping or end at the covered shoulder.
The back of the neckline
The line can continue across the back, resolve into a strap or meet a more conventional rear bodice. Rear and side views show that transition. A one-shoulder neckline does not by itself specify a one-sided sleeve.
Carven and Maze below the shoulder
Carven combines a one-shoulder neckline with an asymmetric waist and close sheath line. Maze uses an asymmetric high-neck wrap and draping above a mermaid skirt. In both, the diagonal at the top relates to a different lower shape. That relation, plus the view from behind, is what gives the one-shoulder cut its individual character.
A diagonal neckline need not mean a one-sided sleeve: the upper fabric may cross the chest, wrap the shoulder or narrow into a strap. The point where the line closes at the back adds another difference between the designs. That distinction helps separate neckline geometry from the arm treatment on each gown.
Is one-shoulder the same as one sleeve?
No. The neckline describes the diagonal upper opening; the sleeve or arm covering is a separate design choice.