Starting with the main development branch after 6.7.0, the X.Org versions are numbered according to the scheme outlined here.
The version numbering format is M.m.P.s, where M is
the major version number, m is the minor version number,
P is the patch level, and s is the snapshot number.
Full releases have P set to zero, and it is incremented for each
subsequent bug fix release on the post-release stable branch. The
snapshot number s is present only for between-release snapshots
of the development and stable branches.
Immediately after forming a release stable branch, the patch level number
for the main development branch is bumped to 99, and the snapshot number
is reset. The snapshot number is incremented for each tagged development
snapshot. The CVS tag for snapshots is "x-M_m_P_s". When
the development branch enters feature freeze, the snapshot number may be
bumped to 900, and a stable branch may be created for the next full release.
The branch is called "x-M_m-branch". The snapshot number is
incremented from there until the release is finalised. Each of these
snapshots is a "release candidate". When the release is finalised, the
minor version is incremented, the patch level is set to zero, and the
snapshot number removed.
Here's an example which shows the version number sequence for the development leading up to version 6.8.0:
6.7.99.1The first snapshot of the pre-6.8 development branch.
6.7.99.23The twenty-third snapshot of the pre-6.8 development branch.
6.7.99.900The start of the 6.8 feature freeze, which marks the creation of
the "x-6.8-branch" branch. That branch is the "stable"
branch for the 6.8.x releases.
6.7.99.903The third 6.8.0 release candidate.
6.8.0The 6.8.0 release.
6.8.99.1The first pre-6.9 development snapshot, which is the first main branch snapshot after creating the 6.8 stable branch.
After a full release, the stable branch for the release will be maintained
with bug fixes and important updates until the next full release. All
snapshots on this branch are considered "release candidates", so the
first is indicated by setting s to 901. The snapshot number
is then incremented for each subsequent release candidate until the
update release if finalised. The patch level value (P) is
incremented for each update release.
Here's an example which shows the version number sequence for the 6.8.x stable branch.
6.7.99.900The start of the 6.8 feature freeze, which marks the creation of
the "x-6.8-branch" branch. That branch is the "stable"
branch for the 6.8.x releases.
6.7.99.903The third 6.8.0 release candidate.
6.8.0The 6.8.0 release.
6.8.0.901The first pre 6.8.1 snapshot.
6.8.0.903The third pre 6.8.1 snapshot, also known as the third 6.8.1 release candidate.
6.8.1The 6.8.1 release.
6.8.1.901The first pre 6.8.2 snapshot.
6.8.2The 6.8.2 release.