Sequential implies that the gears must be selected in order. Older "H" pattern manuals allowed jumping from one gear to any other.
The issue gets a bit more complicated with 3 shaft, dual clutch transmissions. Since the selector mechanism gets more complex.
It also gets less efficient as all twin clutch so called seamless shift gearboxes absorb far more torque than single layshaft boxes.
An excuse to let the driver believe he is changing gear fast, when it can all be done much more efficiently and effectively with electronic control and fully automaticaly.