pls. don't get this wrong: What happens when you try to enter MORE water in a glas, even when it's already filled up completely? Correct answer will be: The excess water flows past the glass.
You can't use more than 100%, because 100% is the technical maximum.
The documentation of plesk states the following: The CPU capacity is specified in percent, and 100% of CPU limit is one core fully used. Correspondingly, 200% is two cores fully used, and so on.
My tests have shown that actually only one core is fully utilized when I enter 100%.