A Variable is a named position — the simplest form of Term.
Term differentiates into three forms: Variable, Function, and Application. A Variable is the most basic — it names a position within a context without doing anything further. Where a Function binds and a Application applies, a Variable just refers.
In the relational derivation, Variables arise because the relational apparatus, now hardened into manipulable expressions, needs positions that can be named and referred to. A Variable is what Reflexive Sequence looks like once it has been formalized: a name for a particular depth of self-reference within a particular context.
Variables can be typed: the Judgement layer assigns properties to Variables within their contexts, constraining what they can refer to.