> But I'm just wondering how this Paragraph tag could be referenced in the document while it was not in the catalog?
Paragraphs with new tags are not automatically added to the Catalog. It's an option when creating one. When pasting in text from another FM document, new tags are not automatically cataloged. For a tag one already in the catalog, doing a Delete Format on it only removes it from the cat. Text containing it is still present and still tagged with the deleted format.
There actually is a command (dialog sequence) that causes every format in use to get added to the cat (and the details of which I don't recall at the moment). Instances of existing catalog formats with overrides get cat entries with new names. Massive catalog pollution results, but it's great if the intent is to clean up overrides and weed out (or adopt) rogues.