"CD" stands for "Critical Dimensions" and the CD "key" has
all of the features that should be dimensionally controlled.
It's not for your benefit, it's part of the "foundry dues" just
like mask ID marks and alignment marks and WAT structures.
CD measurement data is fed back through the SPC system
to keep lithography on-center.
CDs could be in the chip or in the scribe lanes. I prefer to
see them on the chip, "just in case" something goes wrong.
Just like I prefer to have "drop-in" PCMs rather than scribe
linw WAT keys, because any wafer-processing-received
insight is destroyed before you know you needed it.