Aspiration Flow Rate is tendency of the phaco probe of attracting lens particle matter around it. To remember you can associate it with the term “Followability”.
Vacuum is the ability of phaco probe or the tip to be able to hold on to the chopped or fragmented nucleus matter once it has been brought to it. It is closely related to the term “Occlusion” as only the complete occlusion of the phaco tip would allow the vacuum to be built up within the probe and then attain to the specified level. Unless the tip is completely occluded by the nuclear material vacuum will never be able to build up no matter what the setting may be.
These are the two crucial parameters which will determine how efficiently the surgeon would carry out the emulsification of the nucleus material with minimum fluctuation of the depth of the anterior chamber.