I am not entirely sure what it does, but it feels like turning on "linear scale" spreads volume (over 100 levels) much more evenly if that makes sense.

For example:

When I use apple dongle for my iems, I usually gets comfortable volume around 3-5 steps of volume level. But increase in one more step of volume level from this makes it painfully loud without linear scale turned on.

With linear scale turned on I usually end up in comfortable volume around 60-70 steps. And increase in each steps of volume level is not making it painfully loud.

So I get much more flexibility in adjusting volume to taste with linear scale on.

It acts on the volume in a linear way, where the volume scale (decibels) work in a logarithmic way. Meaning you'll get a ton of volume at max, then it'll drop off quite rapidly and from 50% down to 0, it's fairly quiet. You can see this behavior in guitar volume pots, which are also classified as linear or logarithmic

I haven't verified, but what it should do is convert the volume level from logarithmic to linear. I'll explain.

Log based systems in effect take the logarithm of value. This means that if the volume goes up 10x there is still room on the scale.

Linear doubles the sound when the number doubles (2 doubles to 4, 50 doubles to 100).

In specific cases you can find things happening like with a lot scale 1 can be too high. But with a linear scale that same 1 value usually ends up somewhere around 5-10. The downside is that if you have a device that requires the upper end of scale you've lost all volume control, it has become just an on-off switch as the granularity at the top is lost.