What is the role of pcie_clkin constraint

[mayureshw]

What is the role of the following line in normal.xdc in the sample projects? Seems kept commented. When is to be uncommented?

Is pcie_clkin_p the 'system clock' or it has some more specific role?

#create_clock -period 10.000 -name pcie_clkin [get_ports pcie_clkin_p]