Ah, I see. Yes, so the Firkin container type has a gross/fill capacity and a taxable capacity (which can be set on the container type itself, and adjusted as needed). The gross/fill capacity is how much volume will be transferred to it from a vessel (such as a fermenter), and the taxable capacity is the volume for beer duty purposes.
The part-filled option on a product is to allow you to part-fill the final keg/cask when racking. For example, you may rack 20 full firkins and then put 30L in the final one (to save wasting it). This can then be used in your own taproom (or even sold using the special part-filled product). There’s a bit more on this in this post Best way to deal with 'part fill' containers? - #5 by max
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