[Sabardeyn]

Toxaris,
Can't various metadata items be entered and edited via the Metadata Editor icon on the toolbar? Including the creation of unique fields? Or at least chosen from an expanded list of keywords or categories?*

I realize from your comment that no other software may be able to make use of this metadata, but at least if the OP wants to include it (perhaps for later use) the metadata is already present.



*: Sorry, I'm not running Sigil at present and I'm not sure I've used the correct terms. Hopefully the intent is clear.