Highest Astronomical Tide(HAT) and Lowest Astronomical Tide(LAT) are hypothetical extremes in which all tidal constituents are assumed to be in phase.  Meteorological effects (storm surge) are not included in these limits.

John Simpson

Thanks for your answers. John is of course correct - these are astronomical extremes only, i.e. no meteorological effects. For the manuscript, I've decided on the phrase "maximum astronomical tidal range" - in my study it's relevant  in the context of some palaeo proxies which form at, for example, HAT - I therefore need to simulate how this has varied through time.

John is of course right, but in practice HAT and LAT are defined by means of analysing some tide gauge data with suitable software, determining tidal constants, and then computing tidal predictions (say every minute) for 18.6 years. The highest and lowest levels are then HAT and LAT. The UKHO (in the UK case) defines Chart Datum from that estimate of LAT (i.e. a depth below MSL), usually with some rounding to a sensible number of decimal places. See for example page 10 of the Pugh and Woodworth 2014 sea level book.

http://www.skysailtraining.co.uk/tide_definitions_causes.htm