What types of A/B tests can I choose from?

In the A/B test you can only test subject lines.

Legacy editor
There are four different types you can choose from. Each test is an individual part of the message that will be tested.

CONTENT
When you decide to test the content of your message, the A/B test messages will differ by the content only. Anything else, such as a subject line or from-field, will be taken from the source message. If you choose an already existing message for this test, its subject line and from field email address will be replaced by the one from the source newsletter.

SUBJECT LINE
In this test, you can define up to five different subject lines for one message. This should help you decide which one of them is most catchy and generates the most opens.

FROM FIELD
Find out how the from-field that you choose can make a difference in your stats. In this test, you can use up to five of your existing from email addresses, previously defined in your account settings.