The setting limits the number of concurrent web requests that Lucee will process. Note that every proxy or web server (including CommandBox) have their own concurrent request limit. The smallest one in the chain will be the most limiting.
Correct.
It’s more a matter of saving the app from crashing under unbounded load by limiting how much it’s doing at one time.
If you’re not having issues, I wouldn’t touch it. By default, Lucee has no limit, unlike Adobe CF. If you’re using CommandBox, it has a default limit of 30, BTW.
For some sort of API which has very fast requests that only take a couple ms and put very little load on the server, you could allow hundreds of concurrent request. But if your app is doing heavy stuff like cranking out PDFs or something CPU intensive, you may want to throttle it down.
But honestly, I wouldn’t touch it until you’ve had issues AND done root cause analysis to know the issue relates to requests stacking up or dogpiling on the server. FusionReactor will be invaluable in determining this.