The flow characteristics of a control valve is the relationship b/w the flow rate through the valve and the valve travel position between 0 to 100%.
Linear flow characteristics
The change in flow resulting from a fixed increment of valve travel is constant throughout the whole stroke. Linear flow characteristic valves are generally used for liquid flow services and for certain flow control applications requiring constant gain.
Equal-percent flow characteristics
Equal increments of valve travel produce equal percentage changes in the existing flow. Equal percent flow characteristics valves are generally used for pressure control applications.