Explanation of ethernet transformer signals RDCT, TDCT, RXCT, TXCT
Ground means a 0V reference.

Earth Gnd means connect to AC line earth gnd.

When CM noise needs to be reduced, the  terminator R's common are terminated to the 0V at each end with 0.01uF and shield with 1nF or 1nF/0.1uF cap to avoid AC ground loops at line f.

This reduces CM noise, if the units is not mobile and has an Earth ground that differs  at each end.

note the different use of Gnd symbols.

The PHY magnetics has both a CM choke and a differential isolation transformer with a centre tap. 

The CMRR of the CM choke is improved here by use of a 1500pF cap to Tx ground as additional CM rejection filter LC LPF due to impedance ratio XL/Xc of CM choke X rising with f and CM shunt cap Xc reducing with rising f, while not affecting the differential signal.