Ebuilds may have a Gentoo revision number associated with them. This is a
-rX suffix, where X is an integer
—
see File Naming Rules. This
component must only be used for Gentoo changes, not upstream releases. By
default, -r0 is implied.
Ebuilds should have their -rX incremented whenever a change is made which
will make a substantial difference to what gets installed by the package
—
by
substantial, we generally mean "something for which many users would want to
upgrade". This is usually for bugfixes.
For any such revision bump, the new ebuild should be based on the
previous revision to ensure that fixes aren't dropped accidentally.
Simple compile fixes do not warrant a revision bump; this is because they do not affect the installed package for users who already managed to compile it. Small documentation fixes are also usually not grounds for a new revision. In particular, this applies if the package has a substantial compilation time; developers should use their best judgement in these circumstances.
When doing a revision bump, the usual rules about dropping to ~arch apply.
See Keywording on Upgrades.