The following applies to Vector Motion Blur:
Strength is the degree of blurring. A value of 100% will blur the object for the entire distance between one frame and the next. For extra blurring, enter a value greater than 100%.
The following applies to Motion Blur in the Physical Renderer:
Here you define if an object should be rendered with Motion Blur (due to its change in position, rotation or size; this would reflect a No Deformation setting), with additional deformation blur (due to the effect of Deformers; Full Motion) or if any motion blur at all should be rendered.