Physically exclusive clocks are clocks that are defined on the same source point and propagate on the same clock tree. The following figure shows an example where two primary clocks are defined on the same input port.
Figure 1. Example of a Design with Physically Exclusive Clocks
While their overlap is convenient for timing several application modes with one design and constraint database, these clocks and their children generated clocks should never be timed together. The Timing Constraints wizard identifies such clocks and recommends a clock groups constraint to prevent unnecessary timing analysis on the clock domain crossing paths, as shown in the following figure.
Figure 2. Example of a Design with Clock Groups Constraint