This definition of collimating mark might help in the general sense: "An index mark, rigidly connected with the camera body, which forms an image on the negative. This image is used to determine the position of the optical center or principal point of the imagery. Also called fiducial marks." - thefreedictionary.com
"Carbon fiducial marks are formed during thin-film local delamination processes induced either by superlayer indentation forming circular blisters, or by residual stress relief through telephone cord blister formation. Hydrocarbons are sucked into the crack tip during the delamination process, outlining the cracktip-opening angle, which can be used to back calculate thin-film adhesion using either elastic or plastic analyses presented here." - Alex A. Volinsky; Neville R. Moody; Michael L. Kottke; William W.G. Gerberich, 'Fiducial mark and nanocrack zone formation during thinfilm delamination', Philosophical Magazine A, Volume 82, Numbers 17-18, 20 November 2002.