The word billet is not confined to aluminium alloy.
It does not mean "scrap".
It does not indicate any particular metal, grade or physical properties.
It is an ill-defined term for an ingot to be extruded, or short length or slice of cut bar stock.
The latter, typically but not necessarily round, is usually either of a specific length for a particular purpose (including sales-units as by our retailers), or is the bar-end from cutting those pieces.
It is an old, generic word and cannot possibly be used as a trade-name.