It is often useful to refer the reader to other parts of your document; but putting literal page, section, equation, or whatever numbers in the text is certainly a bad thing.
Eplain therefore provides commands for symbolic cross-references. It uses an auxiliary file with extension .aux (and the same root name as your document) to keep track of the information. Therefore, it takes two passes to get the cross-references right--one to write them out, and one to read them in. Eplain automatically reads the .aux file at the first reference; after reading it, Eplain reopens it for writing.
You can control whether or not Eplain warns you about undefined labels. See section Citations.
Labels in Eplain's cross-reference commands can use characters of category code eleven (letter), twelve (other), ten (space), three (math shift), four (alignment tab), seven (superscript), or eight (subscript). For example, `(a1 $&^_' is a valid label (assuming the category codes of plain TeX), but `%#\{' has no valid characters.
You can also do symbolic cross-references for bibliographic citations and list items. See section Citations, and section Lists.