The wearing of masks is at the core of Chosen culture, and their use is controlled minutely by codicils of the Law-that-must-be-obeyed
This table shows how Masking Law works. The ranks of individuals—given along the top—reference a compulsory state of masked [M] or optional state of unmasking [UM]—indexed against a list of ranks of those present—listed down the right of the table
More precisely (quoting from the epigraph that begins chapter 8 in The Chosen):
The Categories of Concealment are: first, the offspring and the consorts of the God Emperor; second, the Ruling Lords of the Great and the Grand Sapients of the Wise; third, other Lords of the Great and the Ruling Lords of the Lesser Chosen; fourth, the remainder of the Chosen and the Wise; fifth, the ammonites of the Wise.
The Protocol of Concealment states that those in a lower category must unmask whenever those in a higher category do so unless this contravenes the second Law of Concealment.
The Categories of Seeing are: first, Lords of the Great and Ruling Lords of the Lesser Chosen; second, the remainder of the Chosen; third, a Lord’s own household or the ammonites of the Wise; fourth, the household of another Lord; fifth, marumaga; sixth, all other creatures.
The Laws of Concealment are: first, that the God Emperor must always remain concealed; second, that the number of a Category of Seeing determines the Category of Concealment in which concealment may be waived; third, that, for the Unchosen, a Category of Punishment is referenced according to how much the Category of Concealment exceeds the Category of Seeing.
Below is a detail from one of my notebooks showing my original scheme for masking law.