Alloy and temper
Most architectural extrusion is a 6000-series alloy, and the temper is what tells you how it will behave. A harder temper machines and holds an edge better; a softer one bends without cracking.
If a section is going to be formed, bent or rolled after delivery, say so when you order. The right temper for a straight mullion is the wrong one for a curved head.
Mill finish, anodized or powder coated
Mill finish is the raw extrusion and it will mark and oxidise. Anodizing puts a hard, integral layer on the surface and keeps a metallic look. Powder coating gives you any colour you like and hides more.
Anodized and powder-coated lengths from different batches can differ slightly in tone. If sections will sit side by side on a visible elevation, order them together and say that is what they are for.
Tolerance and straightness
Long sections bow. It is normal, and it is why straightness is checked on incoming stock rather than assumed. For anything over about three metres that has to sit true against glass, it is worth saying so at the order stage.
Cut to size, or stock lengths
Stock lengths are cheaper per metre and generate offcuts. Cutting to the dimensions on your drawing costs a little more and removes the waste and the labour. On a job with a lot of short pieces, the second is almost always cheaper overall.
