BRASS OR STAINLESS
Many nozzles are available in brass, stainless steel, or a combination of both.
Brass is lowest in cost and transfers heat better for hot melt adhesives. Being softer, it wears faster if the product rubs against it. This may distort the orifice thereby changing the pattern.
Brass is seldom used with food, medical products, perfumes, or chemicals that may interact unfavorably.
Stainless is tougher, wears better, and is more costly.
Some nozzles have a brass body and a stainless steel insert. The insert usually encases the fluid, while the brass results in lower costs and better heat transfer. Often the nozzle used is a matter of personal experience and what you find works best for you.