Multicycle path exceptions must reflect the design functionality. They must be applied on paths without an active clock edge at every cycle. This applies to either the source clock, the destination clock, or both clocks.
The path multiplier expresses in terms of clock cycles. It uses the
source clock when you use the -start option. It uses the
destination clock when you use the -end option. This is
particularly convenient for modifying the setup and hold relationships between the
startpoint and endpoint independently of the clock period value.
The setup relationships always tie to the hold ones. Consequently, in most cases,
the hold relationship also needs to be separately adjusted after the setup one is
modified. This is why you need a second constraint with the -hold option. The main exception to this rule is for synchronous CDC paths
between phase-shifted clocks: only setup needs to be modified.