When doing A/B testing in Marketing Cloud, the options here are to send a certain percentage to Email A, a certain percentage to Email B, then the "Remainder". My question is "what exactly does the Remainder audience get sent"?
The remainder will be emailed the winning email. They will not receive an email during the test period. If set to perform automatically, the email is sent immediately on termination of the test. When set to manual, you can review the winner and test details, change the winner manually if you like, and send the remainder the winning email.
In addition to sfdcfox's answer, something to be aware of (as it's a common question/issue) ....
If you are using A/B test with a Data Extension, then make sure that the data extension does not refresh i.e. the audience change, during the duration of your test.
If you data extension gets refreshed, then the audience will have changed and the 'remainder' send will fail to send.
Always use a static data extension when sending by A/B testing