Taking the example of an air-operated valve, there are two control actions possible:
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“Air or current to open” - The flow restriction decreases with increased control signal value.
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“Air or current to close” - The flow restriction increases with increased control signal value.
There can also be modes:
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"Air or control signal failure to close" - On failure of compressed air to the actuator, the valve closes under spring pressure or by backup power.
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"Air or control signal failure to open" - On failure of compressed air to actuator, the valve opens under spring pressure or by backup power.
The modes of failure operation are requirements of the failure to safety process control specification of the plant. In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed.