A 100% rated breaker can carry its full rating continuously. That means that the continuous load can be held at 100% as well as the non-continuous load.
Conductor sizing isn't covered here, but there is no requirement to derate the ampacity of current-carrying conductors except for temperature or voltage drop considerations. If the table says that a 500kcmil will carry 380 Amps, you can load it all the way to 380 Amps.
