Trimming, ultimately, reduces drag (when talking about a commercial airliner at least). When you trim a light aircraft, you move a tab on the elevator surface in one way or another in order to neutralise the forces felt in the control stick/column. Although this makes the flying easier as you’re not constantly pushing/pulling on the stick or yoke (trust me, it’s gets very tiring very quickly), it also creates more drag.
Now, trimming works differently for a commercial aircraft with something called a Trimmable Horizontal Stabiliser (THS) - note that this is Boeing’s name for it, Airbus call it something else. The THS is essentially an elevator that hinges off another non-moving surface in front of it, all of which is then bolted to the aircraft fuselage. Pretty standard stuff, much like a lot of GA aircraft, and this can be observed on the vast majority of the aircraft in Infinite Flight.
The difference is in how the aircraft is trimmed. Instead of moving a tab on the trailing edge of the elevator surface, the entire unit is rotated relative to the fuselage on which it is mounted. This is the effect that @xsrvmy mentioned is only shown on the 772.
The whole point of moving the entire unit that, for a load of principles of flight reasons, it reduces drag relative to the other ways of trimming. The other reason is that by moving the whole front portion, full elevator authority is retained (i.e. you still have the full range of movement of the elevator, thus giving control over a greater range of speeds, weights, configurations etc).
This whole THS assembly moves about a nominated neutral point, which appears to be taken as ‘0%’ trim in Infinite Flight. The percentage shown is then how much the THS is deflected from said neutral position by. Therefore, 30-40% is perfectly normal and reasonable for cruise.
It’s worth noting that trim does not vary from person to person during the cruise, despite what some others may say. Is does, however, depend on personal preference when on final approach and landing - during this phase of flight, use what feels right.
I hope this explanation helps you understand how the trim system works and why it is represented how it is in Infinite Flight. I realise I’ve gone a bit overkill for Infinite Flight, but it’s interesting to know none the less.