Personally I find these attempts to differentiate between "forward" and "side" slips erroneous and unhelpful. It seems to come down to somebody somewhere's attempts to over-complicate life.
Sideslip angle is the angle between the fuselage and the local airflow, as described in a plane defined by the fuselage and wings. Normally, with the ball in the middle, sideslip angle (or beta) is zero. So a sideslip is any occasion where beta is not zero, and you can usually tell that this is the case when the ball isn't in the middle.
Sideslip can be used to lose height on approach - by making opposite stick and rudder inputs; particularly useful when too high on a PFL.
It is also used as a way of landing crosswind; the "wing down" approach uses ailerons to keep the aircraft tracking down the centreline, and rudder to keep it pointed down the runway. It's still just sideslip within a manoeuvre.
It is also used to enter a spin; which is caused by significant sideslip as the aeroplane is stalled. Again, sideslip within a manoeuvre.
It's used in some other aerobatic manoeuvres.
G