If you are planning to split site visitors into different experiences without setting up an Experiment, or if you want to test a campaign vs "no campaign", you can do that using the Random Percentage Conditions Extension.
First, navigate to the extensions menu under your account dropdown:

Search for and install the Random Percentage Conditions Extension.
This will add a new set of conditions to be used for your campaigns:

Pageview Random Number: Visitors are assigned a random number between 1 and 100 for each pageview. To target 10% of pageviews, use Pageview Random Num 100 greater than 90.
Visitor Random Number: When they first visit your site, Visitors are permanently assigned a random number between 1 and 100 that is persistent across repeat visits. To target 20% of visitors, use Visitor Random Number greater than 80.
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