Gentoo Development Guide
Ebuild Revisions
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.
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.
When doing a revision bump, the usual rules about dropping to ~arch apply.
See Keywording on Upgrades.