Yes, there is a difference!
means that primary and continuous pedaling would be the norm, with occasional, or minimal, motor assist being applied.
indicates that the primary and constant force would be the motor, with occasional, or minimal human power assist being added.
Ideally this would be for a cyclist requiring some degree of assist for greater speed, range ... possibly, merely to help on hills.
This type would be best served with a,
geared hub motor or
a freewheel attached external drive motor or
a (choke!) friction drive motor.
More popular with commuters, restricted capability types, etc.
Designed to continuously use motor power, but capable of better acceleration, extending range and speed by applying pedal assist.
This is a perfect fit for the typical hub motor, efficient, quiet and smooth (it's "drag" when not powered is a detriment that does not come into play).
I was confronted with this difference - dilemma when asked to rebuild adult trikes as mobility devices
Actually, I was considering 3 possibilities.
1. Motor Only - allowing lowered seat, foot platform, gear reduction motor deceleration, reverse mode
2. Motor Assist - Allowing good exercise and mobility with motor capability for greater speed and range while providing the safety net of motor only capability when fatigued.
3. Pedal Assist - gear reduction motor deceleration, reverse mode, with human power assist for hills, speed, range, exercise.