X = Major version. (Across a major version the Plugin-API is stable. e.g. All plug-ins written for version 2.0.0 will still run in version 2.10.32)
Y = Minor version. (Used for releases with new big features and UI changes)
Z = Maintenance version. (Used for bugfixes and small improvements.
N = Installer version. (Used when nothing has changed in the GIMP source code but the installer. Most of the time this is because of new versions of libraries.)