set_clock_latency models board-level clock delays most useful is for I/O constraints,

derive_clock_uncertainty calls out individual set_clock_uncertainty for every clock transfer

derive_clock_uncertainty’s individual calls of set_clock_uncertainty occur when the timing netlist is being updated, which is after all SDC files have been read in.

If the user has set_clock_uncertainty assignments elsewhere in their .sdc files, those assignments will have priority. 

If the user’s set_clock_uncertainty assignments or the derive_clock_uncertainty assignment has the -add option, then the uncertainties will be additive.