HFN vs CTS

What are High fan out Nets, how do they differ from other nets? 

HFN's are the nets which drives more number of load.  

we set some max fan out limit by using set_max_fan out. 

The nets which have greater than these limits are considered as HFN's. 

e.g. Clock, Set/Reset, Scan Enable nets are high fan out nets. 

HFNS, high fan out net synthesis is process of buffering the high fanout nets to balance the load. 

Since load is directly proportional to Delay(Transition Time and load) ,too much load may effect delay numbers and transition time. 

So by buffering this load can be balanced. 

HFN (High Fan out Net) Synthesis is performed during placement stage . 

Clock is a high fan out net, still buffering is not performed over it. Because it is considered as ideal net. 

They are synthesized during CTS separately.  

Since both HFNS and CTS has different targets. 

CTS is mainly targets for ideal Skew ,Whereas HFNS does not.