I have specifications for a positive displacement pump where, in the data table on the last page, the given flow rate is referred to in the column header as 'Open Flow.'
Is this the maximum flow generated by the pump?
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The open flow is the flow rate through the pump where no pressure is developed. That is, if you have fluid coming in and out horizontally, and the input and output hoses are open to the outside air (not a closed system developing pressure. This is the most flow the pump will ever create.
Usually though, we use pumps to either lift fluids vertically, or to pressurize a closed system. In order to do either of these, you need a higher pressure on the outlet side of the motor than on the inlet side. In these cases, the pump will run slower as it has to exert more pressure. The curves on the second page of your link show you for each bar of pressure you need to develop, how much the flow rate will be reduced. In the case of a pressure piping system, you'll know the pressure rise you need directly. If you're using the pump to push fluid uphill, the pressure will depend on the vertical rise (usually referred to as 'head') and the density of the fluid. There will also be frictional losses which may be worth accounting for depending on the length of your piping system. For water, each meter of head is roughly .1 bar, closer to .09 for oil.
Pump flow rates are usually useless without the curves given in the diagrams. The flow rate and the pressure are linked. You will have to find the operating point for your application.
For example, you know the pressure you want to achieve on the pressure side (e.g., hydrostatic pressure) and you know what pressure drop occurs in your pipe. With this information you can the select a pump that achieves that pressure at your desired flow rate. Open flow is the flow rate the pump achieves with a pressure of 0 on the pressure side or, in other words, with a pressure head of 0 m. I think the latter is more comprehensive.
Take Curve 5 from your data sheet, for example; say you want to pump a fluid (water) from reservoir 1 to reservoir 2 and this results in a geodetical height difference of 16 m. In the case of water, 16 m would approximately result in 1.6 bar pressure. This is your static pressure drop. You can then add the dynamic pressure drop resulting from the fluid travelling through your pipes. This is a function of the flow rate. With those information the purple curve can be calculated and then you can see your operating point (blue).
There are some ways to move the operationg point as you wish, e.g., using a bypass.