C1 = cervical
C2 = petrous
C3 = lacerum
C4 = cavernous
C5 = clinoidal
C6 = ophthalmic
C7 = communicating
Location of the markers corresponds with beginning of the segments (except for C1).
As we see our patient has stenosis in the C3 and C4 segments. Also note that there is hypoplastic A1 segment on the right (origin marked with arrowhead) as anatomic variant.
However when reporting the locations of stenosis I also like to give their descriptive names, not only numbers.
See also interesting publication from Medscape
Aneurysms of the Petrous Internal Carotid Artery: Anatomy