Eplain provides the command \definexref for general
cross-references. It takes three arguments: the name of the label (see
section above for valid label names), the value of the label (which can
be anything), and the "class" of the reference--whether it's a
section, or theorem, or what. For example:
\definexref{sec-intro}{3.1}{section}
Of course, the label value is usually generated by another macro using TeX count registers or some such.
\definexref doesn't actually define label; instead, it
writes out the definition to the .aux file, where Eplain will read
it on the next TeX run.
The class argument is used by the \ref and \refs
commands. See the next section.