Traffic percentage for a test is the percentage of your website visitors you want to involve in the test. Depending on the percentage assigned, VWO randomly includes your website visitors in the test. This feature is helpful if you want to run the test on a small number of visitors initially.
For example, if your website receives 100 visitors a day and you want to run the test for just 20 visitors, specify the percentage as 20%.
To assign the traffic percentage to an existing test, navigate to Settings > Others and move the slider from left to right or manually enter the desired percentage in the text box.