In general, a softer bullet swages into the bore more easily, reducing start pressure and causing the powder to take longer to peak. So I think what Ranier means is their starting load is half way between minimum and maximum jacketed bullet data, and their maximum load will be expected to exceed the maximum load for jacketed data.
By way of confirming that, I had a long talk with a Speer tech last year about their Gold Dot bullets which are, he said, definitely plated jackets, as he, personally, had run the plating machine. He also said he's found load data for Gold Dots with starting loads above the maximum jacketed loads, and many more that overlap. For this reason, when you look at Alliant data created with Gold Dots, you should not assume it is right for jacketed bullets.
Everything here hinges on how the plating is done. There is more and more of it. Hornady has a process for plating rifle bullets in actual gilding metal rather than soft copper, which they do with some newer match bullets because it allows them to make a virtually perfectly uniform jacket thickness all around. That said, I've bought thin-plated cast bullets that you couldn't even crimp lightly without cracking the plating on them. For all practical purposes, these were copper-colored cast lead bullets. They even had lube grooves that were filled. They needed to be fired with lead bullet loads.
Low loads: Generally speaking, bore friction is a much smaller consumer of powder energy than bullet acceleration is. Bullets getting stuck in barrels usually occurs either because the powder was way too slow for the bullet mass, which can cause it to extinguish (see Norma manual for an explanation of the effects of recondensing gases evolved off the powder but not consumed in combustion; this can drop pressure dramatically and cause extinguishing). Under-loading 296/H110 in magnum pistol cartridges has been known to produce this effect, but so can any slow powder under the right conditions). The other cause is pressure so low it can't keep the brass expanded against the chamber wall and the resulting gas leakage around the case and out of the breech drops pressure to the point the bullet never picks up enough velocity to coast the rest of the way past barrel friction and out of the barrel. These are typically super low loads.
In the case of the thicker plated bullets, like the Ranier and the Gold Dot, I think the starting load recommendation is high both to cover themselves with slow powders that aren't safe to load down much, and also just so you don't burn a lot of powder getting to maximum. Bore friction should not be the issue. While the softer copper will have higher friction with steel in a standard sliding test, in a gun bore friction is partly due to the bullet material springing outward against the bore, and the jacketed bullets spring hardest. The other part is due to pressure upsetting the bullet outward against the bore. This one is the main one and is the reason copper fouling in a high power rifle is always an inch or two in front of the throat: that's where the bullet is when pressure peaks and upset outward against the bore is at maximum. So the softer copper will likely have a lower peak pressure limit (probably apparent nearer to rifle pressure, though), but need a higher start pressure for slow powders.
That upset and pressure distortion are the reasons there are "velocity" limits on lead bullets and plated bullets. There really isn't a velocity limit: that assumes a standard barrel length. It's really pressure limits that are the problem. Too much pressure causes distortion that messes with accuracy as well as increasing metal fouling. You can use a longer barrel to get more velocity at the same pressure and the bullet won't care about that. Peak pressure is the main culprit, and Richard Lee's Modern Reloading has a good bit about this. Lead bullets get distorted and lose a lot of lead against the bore when pressure is too high, and soft copper plating will do the same thing, but at a higher pressure. A lot of work has been done with alloys and lubes and gas checks to get them up to high power rifle velocities, and this has been done (see Veral Smith's book, for example). But it takes some effort.
The way to tell how low you can load the plated bullets is to use a chronograph to watch for velocity variation to because erratic; a jump in SD as a percent of the MV. If it does, you are getting too low to count on never sticking a bullet in the bore. But with a fast powder like Bullseye, for example, this will be at a pretty low velocity
Thoughts?