Turbine oil circuit

Power oil refers to the fluid that drives the pistons of hydraulic actuators. Quick-shut oil is the fluid used to open the quick-shut valve and overcome the force of the spring. Secondary oil is a pulsed fluid that emerges from the outlet of the main oil pump, passes through a throttle, and is used to control the up and down movement of the spool in the control valve. Start-up oil is the fluid used to press the piston of the quick-shut valve firmly against the closed position before opening it, thereby ensuring that the valve remains sealed. In some units, start-up oil also serves as the fluid for resetting the emergency shutdown device. Test oil is used during normal operation to prevent the quick-shut valve from sticking, by periodically moving its stem using this test fluid. Return oil includes lubricating oil return fluid as well as fluid returned to the control and protection systems; it may go into a separate return tank or directly into the inlet of the main oil pump, and can be either pressurized or non-pressurized

These things cannot be explained in just a short time; I suggest you find a book on steam turbines to read