If you have 4 CPUs (or cores) and each one was at 100% load, your System CPU would be 400% and your Normalized System CPU would be 100%.
Process CPU is the amount of CPU time being used to service the MongoDB Process and all operating system calls that it makes.
“User” is the portion of CPU time being directly used by the MongoDB process.
“Kernel” is the portion of CPU time that the Operating System is using to service operating system calls.