How should I interpret the symbol "Rg" in mathematical logic?

Question

I am reading Kit Fine's paper Semantics for Quantified Relevance Logic (1988), which gives a model theory for quantified relevant logics. In the first technical section of the paper a model is defined as follows:

A possible model $\mathfrak{U}$ is an 11-tuple $(T,S,D,l,\cdot ,-,\geqslant , \uparrow , \downarrow , \to , \phi )$ where:

1. $T$ (theories) is a set;
2. $S$ (saturated theories) is a subset of $T$;
3. $D$ (relative domain) is a function from $T$ into sets (we use $\mathfrak{D} $ (domains) for Rg($D$), $I$ (individuals) for $\bigcup D$, and $\approx$ (domain equivalence) for $ \{ <t,u> \in T \times T: D_t = D_u \} $);

I am confused about the notation Rg($D$), which as far as I can tell is not explained elsewhere in the paper. How should I interpret this string?

Answer 1

Long comment

It is the range of the function $D$: $\text {Rg}(D) = \{ s \mid ∃t(tDs) \}$.

See page 32: "The function $D$ takes each theory (set of statements) $t$ into its ontology or domain of individuals $D_t$."