>> Yes, the term "flag" carries certain baggage from CS history, but I fail to see an argument to avoid a domain with two values, in general. <<

If it is really a domain, I have no problem. I might have problems
with a domain that has only one value, tho. The problem is that a
flag is a computation, predicate, etc. that needs to be derived over
and over from other values. To describe a box, would you store
(length, width, height) then add a column for volume? Of course not
-- it is redundant. Volume by itself would also hide information
about the three variables that went into computing it.