[s0xxx]

I am by no means a NetBSD guru, and in stead of first trying to explain it, I would rather pass you a link to an official document that explains it very well:

Maintenance branches

Starting with NetBSD 2.0, maintenance branches come in two flavours:

• Maintenance branches which will develop into the next minor release, which we call stable branches, and which is reflected in the version designation along this branch, e.g. 6.0_STABLE, which will evolve into 6.1. The corresponding CVS branch is the same branch that was created from NetBSD-current and which gave rise to the corresponding previous major release.
• Maintenance branches which will give rise to any security/critical release. These are created from each major and minor release. This is what we refer to as the security/critical branches. Because the interval between application of fixes and the tagging of the corresponding release, there is no separate "branch version designation" for these branches, instead the version number will jump from e.g. 6.1 to 6.1.1. Versions in between used to be called e.g. 2.0.0_STABLE, but since the NetBSD 3.0.2 and 3.1 releases they are called e.g. 3.1.0_PATCH, to distinguish them more clearly from the above mentioned stable branches.

What you will find on a stable branch is the last release (major or minor) plus whatever bug fixes and enhancements which will be going into the next minor release, pulled up from the NetBSD-current development branch. For example, if the latest release is 6.0, the CVS branch for it is "netbsd-6" which can be thought of as containing an alpha version of the following 6.x releases.

The security/critical branches only receive fixes for security problems or other critical problems. The aim is to provide such fixes but at the same time minimize the set of other changes which would otherwise get dragged in if one were to update along a stable branch. For example, if the latest release is 6.0, the CVS tag for the corresponding security/critical branch is "netbsd-6-0", and will give rise to 6.0.1 and any following 6.0.x security/critical releases.

The maintenance branch(es) can be considered an easy way to get the most up to date fixes for a given release.

There are daily updated snapshots of the latest maintenance branches, available via both CVS, FTP and SUP. The directories pub/NetBSD/NetBSD-release-6/ and pub/NetBSD/NetBSD-release-5-0/ contain the extracted sources plus weekly updated tar files of both the 6.0 and 5.0 release branches respectively. These files are created in a similar manner to those in the /pub/NetBSD/NetBSD-current directory.

http://www.netbsd.org/releases/relea...ml#maintenance