Rosette is a french word meaning decorative round or looking like a rose. AKA a chrome escutcheon.
I don't think he means an escutcheon because he uses the word escutcheon later in his sentence to refer to an escutcheon.
We are talking about a wall mount two handle faucet with 8" centers(typically) mounted to the back wall of a stainless 2/3 bay sink correct? If so where would he need escutcheons? All of those types of faucets I have ever seen all come with the eccentric union fittings that have an escutcheon built in.
I looked up sink elbow and I think he is referring to what we would just call a "1/2" sweat by flanged fip adapter. But as has been stated, even with a close nipple you still have a gap on stainless. We typically set the sink, mark centers, pull the sink, mount drop eared elbows to the wall, and then set the sink/faucet using brass nipples. This also helps keep the sink anchored to the wall so the caulk joint at the top edge doesn't get ripped apart letting sprayed water drip behind the sink.
We also usually drill holes in the top corners of the backsplash of the sink and screw it directly to the wall too. If you think that is over kill remember that this is a commercial application and we get to charge significant markup on a 1000$ sink and 400$ faucet, the customer expects it to be solid for decades.