Xylene is a hydrocarbon with 8 carbon atoms that contains an unsaturated aromatic ring structure, plus two additional carbon atoms. Essentially benzene plus two extra carbons.
In a refinery, xylene is valued as a source of octane for gasoline blending. However, in gasoline xylene content is also constrained by a limit on total aromatics content, because aromatics contribute to smog formation when lost to the atmosphere through evaporation.
Xylene is also an important feedstock to the petrochemicals industry, so many refiners produce and extract xylene to sell as a product.
The primary source of xylene in a refinery is from the reformate produced in the reformer.