What is the difference between an impurity and an inclusion in the aluminium industry

What is the difference between an impurity and an inclusion in the aluminium industry

From my industry experience at a sheet rolling company, we did use the term inclusion for everything that was not supposed to be part of the final microstructure. Not mentioning cracks here.
You could have constituents / intermetallics (like α-Al(Mn,Fe)Si) which always form during the direct chill casting operations, because of limited solubility of elements in the melt.. The goal is to reduce their number and size again during the rolling operations, so they will not create weak spots in the metal. They are brittle and break up into sharp pieces, which can act as local stress riser. Usually you aim for sizes smaller than 10µm, otherwise you get in trouble in later forming operations.
Other sources of undesirable features in your microstructure can be the result of your melting operations. This can include oxides that form in melting and holding furnaces, and are not completely skimmed off or filtered out. Another problem could come from agglomerated grain refining elements that are added to the melt. This would lead to trails of titanium boride in your metal. Bad melt handling and alloying practice, can lead to inhomogeneous chemistries along the ingot.
Impurities would refer to substances or elements present in the metal, that would not necessarily occur during operations or are part of the desired chemical composition. This could be pieces of refractories (ceramics) from the furnace insulations or launder systems. Or others substances that made it through the filters (or could be even broken bits from the filters).