Hello & Welcome to JA,
This is an oil seperator.Here is how the system works.
The crankcase ventilation system on the 6600 Duramax engine vents the crankcase gases to the atmosphere. Located in both valve rocker arm covers are diaphragms to control the venting of the crankcase gases. As the pressure of the crankcase gases increase they overcome the spring holding the diaphragm in the close position. If a vacuum situation arises in the crankcase the diaphragm closes the port in the valve rocker arm cover. Closing the port will prevent unfiltered air to enter the crankcase.
The crankcase gases travel from the valve rocker arm covers through hoses to the crankcase ventilation system oil separator. The oil separator, separates the oil mist in the crankcase gases. A hose is attached from the bottom of the separator to the crankcase. The oil is returned through the hose to the crankcase.
After the oil mist is separated the crankcase gases are released to the atmosphere through the hose connected to the separator.
No routine maintenance is required to the crankcase ventilation system.