  The International Height Reference System (IHRS) adopted by the IAG in 2015
contains two novelties. First, the mean-tide system is adopted for handling the
permanent tide. Second, the reference level of height system is defined by the
equipotential surface where the geopotential has a conventional value
W0=62636853.4 m2s-2. This value was first determined empirically to provide a
good approximation to the global mean sea level and then adopted as a reference
value by convention. I analyse the tidal aspects of the reference level based
on W0. The W0 is by definition independent of the tidal system adopted for the
equipotential surface, but for different tidal systems, different functions are
involved in the W of the equation W=W0. I find that the empirical determination
of the adopted estimate W0 is inconsistent from the viewpoint of tidal systems.
However, the adopted estimate and the consistent estimate from the same data
round off to the same value. I propose a simplified definition of IHRF
geopotential numbers that would make it possible to transform between IHRF and
zero-tide geopotential numbers using a simple datum-difference surface. IHRF
should adopt a conventional (best) estimate of the permanent tidal potential
such as is contained in the IERS Conventions and use it as a basis for other
conventional formulas. The tide-free coordinates of the ITRF, and tide-free
Global Geopotential Models are central in the modelling of geopotential for the
purposes of the IHRF. I present a set of correction formulas that can be used
to move to the zero-tide model before, in the middle, or after the processing,
and finally to the mean-tide IHRF. To reduce the confusion around the multitude
of tidal systems I propose that modelling should primarily done in the
zero-tide system, with the mean-tide potential as an add-on.
