state file

An XML file which patcher clients maintain for each target.

upgrade path

A specification for upgrading to the current version. Each released version should have an entry in the versions file.

versions file

An XML file that patcher clients download via HTTP. The current version name is listed, and for each previously released version, it contains an upgrade path to the current version.

The idea is for clients to check this file before launching the game, and only launch the game executable if the current version exists, otherwise it applies the Upgrade Path specified in the versions file.