The primary clock defines a timing reference for your design. The timing engine uses it to derive the timing path requirements and the phase relationship with other clocks. The calculation of their insertion delay begins at the clock source point. This point is the driver pin/port where the clock is defined. It extends to the clock pins of the sequential cells to which it fans out.
For this reason, it is important to define the primary clocks on objects that correspond to the boundary of the design. This ensures accurate computation of their delay and, indirectly, their skew.
The following sections describe the typical primary clock roots.