The annulus designation between the production casing and next outer casing. The letter designation continues in sequence for each and every outer annular space encountered between casing strings up to and including the surface casing and conductor casing strings.
Source: API RP 90, Annular Casing Pressure Management for Offshore Wells, Upstream Segment. First Edition, August 2006. Global Standards
Annuli between the production casing and the previous casing string.
Source: NORSOK D-010, Well integrity in drilling and well operations, Rev. 3, August 2004. Global Standards