Re: Flushcup ??. Forgive my ignorance
Flushcups allow for swivel rotation of your sling, quicker attachment and release then swivels, which can be nice for off dominant hand shooting, etc. Having a side mount instead of standard bottom sling mount allows the rifle to sit more steady when slung over shoulder instead of flipping side to side especially with a large scope. I also find that swivels tend to loosen easier and are not as durable for me.
But it comes down to preference, I'm sure there are others who prefer something else and have had bad experience with flushcups.