Clock networks are internal clock network pins used as a clock source. With this network type, you can set the clock constraint on any pin in the clock network. You may want to do this to eliminate clock network pessimism by short-cutting a reconvergent combinational logic on the clock network (as shown below). Clock network pessimism triggers an overestimation of the clock skew, making the timing analysis inaccurate.
Source Being Set Within a Clock Network