Q. Why are pump characteristic curves used sometimes presented in
dimensional form, while at some other times they are presented in
dimensionless form? What are their respective purposes?

Q. When are pumps used in parallel and when are they used in series?
How to the H-Q curve of pumps combine when the pumps are used in
combination?

These are obviously questions from your homework or from an exam. If
you do not know the answers to such basic questions, may I suggest you
quit partying and start studying? Or switch your major to Business.

The reason is that if you use only water, the density and viscosity is
known. So the curves can be given in dimensional form. If you want to
use it for any fluid (i.e different viscosity and density), you can
use the dimensionless form.

You can find more info on the following link:
http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=ReadPumpCurve.htm

No the questions are not to homework or to an exam and the reason I do
not know the answer is not due to partying. I am reviewing some things
and those were a couple of questions asked that I did not know the
answers to. Im not a Hydralics major.