Bench range is the pressure needed to open or close the valve when the valve is not expose to the process fluid (not yet installed or no fluid work on it). Just simply ask the vendor to provide this data on 20% opening and 100% opening for example, 5 psig to open 20% and 15 psig to open 100%.
This information is needed later on in the pre-commissioning or commissioning phase in the project. Because in that phase someone will test the control valve manually without the regulated air supply connects to the positioner. It will be a direct connection between the regulated air supplies to the actuator. So, they need this information to test the valve at some % opening.