These are the extras that you can track quantities of.
"inventory-backed" as we have been calling them. Look and behave like an extra from an online booking point of view, but are underpinned by actual inventory.
Example use-case
You sell a tour that has optional bike rental.
Sell the 'tour' as the core 'product' - with the optional bike as an extra - but underpinned by your actual inventory.