All unquantified variables in this entry should be taken as tacitly universally quantified; hence all axioms should be taken as universal closures. No axiom requires more than three quantified variables; hence a translation of first-order theories into relation algebra is possible.
In this passage, any undefined variables are treated as universally applicable. Consequently, all axioms must be considered universal statements.  Furthermore, no axiom requires more than three quantifiers to express its meaning, making it possible to translate first-order theories into relational algebra.