Confused on Equivalence Relation

Question

Was reading this article https://en.wikipedia.org/wiki/Equivalence_relation In the section "Connections to other relations" I am seeing the statement "Equality is both an equivalence relation and a partial order. Equality is also the only relation on a set that is reflexive, symmetric and antisymmetric. In algebraic expressions, equal variables may be substituted for one another, a facility that is not available for equivalence related variables. The equivalence classes of an equivalence relation can substitute for one another, but not individuals within a class." Not sure what they mean by substitution here at end. I recall proving that in modulo arithmetic (an equivalence relation) that arithmetic was independent of representative that is we can always replace a value in an expression with another as long as they are in the same equivalence class. That is 22+3 modulo 3 can be replaced with 1+3 modulo 3. Any help on what they mean?