On Pro and above, you can randomly segment your orders using the campaign rule "Send for order numbers ending in" to test various email features and see what gets a better open rate.
This rule allows you to select any digit(s) 0-9 to target a random segment of your buyers. Selecting any five digits will send the campaign to approximately 50% of your buyers; selecting any two digits will send to approximately 20% of your buyers, etc.
For example, you can test whether sending in the morning or evening gets a higher open rate from your buyers by adding this rule. First, create a campaign and add this rule with 0, 1, 2, 3 and 4 selected. Set the campaign timing for the morning. Then, create another campaign using the same template but with 5, 6, 7, 8 and 9 selected and send this one in the evening.
You can also test different template variables using this rule, such as subject lines. To test two different subject lines, create two templates with different subject lines and a campaign for each. Add this rule to each but with opposite digits selected, and make sure all other rules and timing options are the same.