While labels are provided by Confluence to group pages by an arbitrary sequence of characters, normalized to lower case letters, projectdoc tags allow to do the same but without making the tag names visible to the label system.
So if you want to tag a document publicly, use labels. If you want to add an arbitrary discriminator, only shown on the page, use tags.
Querying documents by labels and tags is supported by the Lucene keyword Tags.
Example Lucene Query on Labels and Tags
Tags = "My Tag" AND Tags = "my-label"
If you use the Tag List Macro, the Confluence labels are also rendered in the tag row of the document properties table. This allows you to provide a documentation for your Tag.
