What does the 26/6 mean in a pump rating?
It has been a few years since I looked at specs on one but the two numbers are identified as Q/H or (m3/h by m).  (m = meters)
To my understanding Q is volume of flow in m³/h (25), so how many cubic meters of liquid can flow in a hour period.
With H is delivery head in m (height) which is the height you can pump a liquid to straight up.
These are the basic description for standard values.  In an exact case to determine what values are needed in a specific application would be based on all the variables such as pipe sizes in the entire system, desired pressures, suction tank/source levels and more.