The versions.ent file exists for documents such as the Reference Manual that exist in multiple versions (for MySQL 5.0, MySQL 5.1, and so on). Every version-specific document directory will have a versions.ent file. Each versions.ent file contains definitions for the same set of entities, but the entity values are specific to one version of the document. For example, the versions.ent file for the MySQL 5.1 Reference Manual looks like this:

<?xml version="1.0" encoding="utf-8"?>
<!--
  This file contains only entities for which the value varies per
  version of the manual.
-->
<!ENTITY title-refman-previous "MySQL 5.0 Reference Manual">
<!ENTITY current-series "5.1">
<!ENTITY previous-series "5.0">
<!ENTITY next-series "5.2">
<!ENTITY current-version "5.1.18-beta">
<!ENTITY current-maturity "beta">

As the current version of MySQL 5.1 changes, we update the value for the current-version entity, and possibly the current-maturity entity.