There have been quite a few models of Waterpiks over the years, with different features and capabilities. There also seems to be a good deal of confusion and controversy around how to (and how not to) use them.

I went for a long time using a Waterpik only occasionally, and mostly when I was after something in particular like an irritated spot between my teeth. I had a very good dentist (now retired) who used to caution me to keep the pressure down. He also mentioned the possibility that if I tried to flush something like, say, a poppy seed out of a gum "pocket", I just might end up driving it in deeper and causing more problems.

A hygienist who'd worked with that dentist for many years (and whom I felt exceptionally comfortable with) left during the pandemic. Not long before the dentist retired, he started using this other, very enthusiastic and assertive hygienist (I won't say "bossy" or "aggressive," though I confess those terms have occurred to me on occasion). The new hygienist instructed me to use my Waterpik more, and not to reduce the pressure. I protested that the dentist had said to keep the pressure down. They conferred, and he came back with the word that ... we apparently hadn't been talking about the same thing or something. I still suspect that he caved to the hygienist, but we never had a chance to continue that discussion.

Later that same hygienist (working for the new dentist in the same office) ordered me to use the Waterpik daily, at maximum pressure, and with a full tank of water each time. I did some research online and came across at least a few sort-of-reassuring articles:
How Safe Is A Waterpik To Use On Sensitive Gums?
Safety of a Water Flosser: A Literature Review | February 2015 | Compendium
I did start using the Waterpik daily, and gradually ran the pressure setting up to the maximum available on that particular machine. Right after a cleaning (an especially aggressive one, I thought) by that same hygienist, I discovered a spot between two teeth that would regularly hurt like blazes when I'd aim the water jet at it. I went back and asked the dentist about it. She just said it looked "a little pink" but she didn't see anything wrong. Over the next few weeks I developed more and more such sore spots until, six weeks along, I decided enough was enough. I took the Waterpik pressure down a couple of notches and over the next few weeks, my gums gradually felt better and better.

The next time I saw my periodontist, I told him about my recent Waterpik experiences and asked what he'd recommend. At first he seemed eager to avoid getting into a fight with the hygienist, but eventually suggested that it wouldn't be a good idea to traumatize the tissue so yes, I should turn the pressure down if it hurt. He also recommended aiming the water jet "perpendicular to the tooth" (as opposed to "up" or "down").

When I saw the new dentist again (for a crown this time, not a cleaning) I mentioned that I'd had to turn down the pressure on the Waterpik. It became clear that she had no idea what specific model of Waterpik I was using nor what its pressure range was. I thought back to some of the studies I'd read about, that had concluded that use of a (generic) water flosser didn't cause any gum damage, and it occurred to me that the authors also apparently hadn't specified what kind(s) of Waterpik(s) their patients had used, nor at what pressure settings. I'm not even 100% sure that they knew.

My "main" Waterpik has a valve at the end of the hose where the tip inserts, with a choice of about five flow (and pressure) settings. I've been using it in the middle position, two notches down from the top. My battery-operated "travel" Waterpik (which I got only a few months ago) has a switch with "High" and "Low" positions. I use it on "high" (and replace the batteries when "high" starts feeling a bit low). With both machines, I mostly squirt the water through the spaces between my teeth, not against the gums and not into the pockets. By now I'm pretty comfortable with those settings, and everyone who cleans my teeth comments that my hygiene seems to be good and they're finding very little "hard stuff".

I haven't run into the "aggressive" hygienist again; that could very well be just the luck of the draw but I like to imagine that my new dentist has had the good sense to intentionally keep us apart.

I've seen other models of Waterpiks offered for sale that have a dial with numbers from 1 to 10. I've never tried one, and probably won't until one of mine quits. The one that Tart Cherry Jam described sounds different still. Anyway, my point is that anything you figure out about one model of Waterpik at one setting, is going to apply differently, maybe very differently, to another model at a different setting.