Implicit/internal field names
When plugin meta data comments get extracted into a hashtable ($meta.*), some common attributes might get populated. Either implied from the filename, or from predefined defaults.
# id: basename
With e.g. the depends: field plugins can reference each other. And they typically just list basenames, not full paths. Because the point of PMD is to abstract filenames from plugin names.
# fn: src/plugin/featurename.rb
# id: featurename
Albeit one could use a literal # id: override in scripts, it's probably best to avoid ambigious basenames. For the scope of one application API all plugin identifiers should be unique.
# fn: .../src/plugin/filename.rb
When a script gets parsed, it's often necessary to retain its exact location. For brevity, I'd usually store this in an internal field named $meta.fn
# comment: …
… … … … … … …
The remainder of the topmost comment block (after all PMD key:value fields) is meant to contain a longer description. And for my implementations I usually keep it as $meta.comment (albeit also used $meta.doc in some extractors.)