Peter wrote:

3. Why are they always specified as this or that Bar rating when this is
meaningless without a corresponding flow at given pressure.

The output pressure of fluid pumps is traditionally quoted in head of
water - the head of water the pump will support with a zero pressure input.

Picture your pump sitting on the ground. The inlet is connected to a
large tank of negligible height sitting next to the pump. The outlet is
connected to a clear vertical tube. Turn the pump on. How far does the
water go up the vertical tube before it stops? That's the head. 30' of
water ~ 1 bar.

The other important property is pumping rater - this changes as the back
pressure changes.

--
Grunff