On the Experiment summary page you find the option to change the distribution of an experiment. It can look like the image below:
The distribution allows you to control what percentage of visitors you like to test. In the example above you can see that 25% of the visitors will not participate in the experiment and will see the original version.
Their behavior will not be monitored. The test group of 75% is equally divided over 37.5% and 37.5% (displayed here as 37%).
The distribution has icons when hovered over them to preview (opens in a new window) or pause a variation. The most left variation is always the original.