In previous versions this was set to false, or didn't exist. This caused all kinds of confusion on the test functionality sending out two messages, one in HTML and the other in Text. I still think it makes sense to keep this OFF to check that both versions come out fine, but to avoid confusion, it is now set to true.

This means that when sending a test email, the version being sent will be the one matching the profile of the address the test is being sent to.