According to pkg-annotate(8) it's possible to use pkg annotate to basically add your own tags to packages. However, the entire pkg(8) documentation appears to not contain a single example of how to actually use annotations. For example, after doing e.g.
pkg annotate -A packagename foo bar, how can one use pkg query to obtain a list of packages that have the annotation foo set to bar? I cannot find a single example anywhere in the pkg documentation.