There isn’t yet much ArduSub-specific documentation related to position-aware flight modes - this comment describes the relevant context.
This isn’t something I’ve gone looking for - my point was more that ArduPilot’s positioning and control systems are designed to handle a variety of scales, where generally if you have sufficiently fast and accurate positioning sensors you should also be able to autonomously command comparably precise positions and manoeuvres.
Even MAVLink’s GLOBAL_POSITION_INT message supports spherical (horizontal) and vertical resolutions of ~1mm, and LOCAL_POSITION can conceivably be even higher resolution, so positioning accuracy issues are much more likely to be from time delays, sensor inaccuracies, and a lack of actuator fidelity than from a lack of firmware support for storing and handling precise positions.