What is this

Smoke tested loads of points on the engine and this is the only point where smoke is coming out the round cap thing with three slots what is the purpose is it a inlet or outlet valve. The diagnostic is egr circuit a low power fault taking off the pipe from the air filter box and removing the small pipe from the rear it is very oily any help appreciated guys thanks

The round thing with 3 slots is a safety valve, which a thin diaphragm is supposed to burst if the crankcase suffers from sudden high internal pressure.
It should be sealed up, unless it's blown, in which case the top cover needs replacing. It most definitely shouldn't allow engine gasses to escape in normal operation.

The codes suggest the EGR valve has failed. When the EGR valve is replaced, the new valve needs calibration with an air path calibration routine, or it'll fail again very quickly.