Generally, the semi trailing arm has a pivot axis angle around 20 to 30 degrees.

If both pivots are on, or very close to, a line at right angles to the vehicle centreline, then this is referred to as Trailing arm
The rear suspension on the original Austin/Morris Mini is a typical example.

If both pivots are on, or very close to, a line parallel to the vehicle centreline, then this is referred to as Swing axle

Note that the original Volkswagen rear suspension, which is usually called Swing axle is actually more of a swing axle as the pivot axis angle is around 60 degrees. The trailing arm which connects to the torsion bar acts as the outboard pivot, with the driveshaft universal joint acting as the inner pivot