Since you question I'll try to shed some light on it for you.
It all starts with a 100% rated OCPD such as a molded case circuit breaker that is UL listed for application at 100% of its rating. These breaker will have a solid state trip unit and usually will be at least a 400A frame if not larger.
100% rated breakers aren't any better than the its common counterpart. The only difference if that they have been UL list for 100% application only when used in an enclosure of installation that is also listed with it for 100%.
When you are so inclined to apply a 100% breaker and read the instructions carefully you will find that it must be applied with 90degC rated cable, the cable of which is applied at 75degC. Remember that cable must be sized to carry the load and breakers then protect the cable.
Normally, the cable is sized 100% of the non-continuos load plus 125% of the continuous load. The cable size selected must carry that load. The breaker is sized to protect the cables rated ampacity that is often more than the calculated load because the breaker rating is based upon the cable rating and not the load.
When applying 100% rated breakers it is 100% plus 100% of the continuous and non-continuous loads. You select the cable size based upon 75degC and you will note that it gives you the opportunity to select a smaller cable.
Smaller copper cable means lower cost. BUT, that cable MUST BE RATED 90degC and not 75degC. This may also give you the opportunity to step down a breaker frame size depending upon what cable size that you end up with and you get lucky. The next smaller frame size can also reduce cost.
So applying cable at 100% is a bit more than that.
I trust that this sheds some light on the application.
You may also not that 100% rated breaker have electronic trip units. The standard breaker will have mechanically thermal magnetic as well as the available identical electronic trip unit. The electronic trip unit does derate with ambient temperature that makes it ideal for 100% rated breakers. Electronic trip units don't make a better breaker but they do provide the ability to apply the LSIG features, digital read outs, communication, etc. Electronic trip units provide a lot more capability to coordinate.
To be honest, we in OEM sales are very intrigued by those who think they are getting a better breaker because it's 100% rated. We love to collect that extra money. I was a Sales and Application engineer for 18 years and am now a Sales Engineer for a dry type transformer manufacturer and we still supply 100% breaker because they are specified knowing that they can't be applied per their 100% UL listing. The customers don't have a clue. They can only be applied, as would be a standard breaker.
