A projection formula in which vertically drawn bonds are considered to lie below the projection plane and horizontal bonds to lie above that plane. Thus for the molecule Cabcd:
F02391.png
Sources:
Blue Book, p. 484 (https://www.acdlabs.com/iupac/nomenclature/)
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2208 (https://doi.org/10.1351/pac199668122193)
See also:
White Book, 2nd ed., p. 128 (https://iupac.qmul.ac.uk/bibliog/white.html)