In libraries or packages I often see something like 0.5.4+6 or maybe 1.12.4+2, etc. I know the first number is the major version, the next one minor version, the next one maybe build number or revision. But what does the +2 or +6 signify?
Usually it is used to provide some metadata / build metadata (eg. a build number or date).
For more detailed info, see the Semantic Versioning spec.