question about set_input_delay

The following list describes the options for the set_input_delay command, anybody tell me when shall we specify the -rise|-fall option?

-clock <clock name> 	Specifies the source clock. 	 
-clock_fall 	Specifies the arrival time with respect to the falling edge of the clock. 	 
-rise | -fall 	Specifies either the rise or fall delay at the port. 	 
-max | -min 	Specifies the minimum or maximum data arrival time. 	 
-add_delay 	Adds another delay, but does not replace the existing delays assigned to the port. 	 
-reference_pin <target> 	Specifies a pin or port in the design from which to determine source and network latencies. This is useful to specify input delays relative to an output port fed by a clock. 	 
-source_latency_ included 	Specifies that the input delay value includes the source latency delay value, and therefore any source clock latency assigned to the clock will be ignored. 	 
<delay value> 	Specifies the delay value. 	 
<targets> 	Specifies the destination ports or pins.