When i build emails for A/B Tests i add ' A/B test' in the subject line within the email builder to remind myself this email should be tested.
When the A/B test is set up and launched - the original subject line 'A/B test' is overwritten, right?
Broadly yes, but there's some differences to how, depending on exactly how you set your test up (and whether you're doing A/B testing or Champion/Challenger testing - if you're working in an Email Program it'll be A/B testing, if not, Champ/Challenger).
If you overwrite the original subject line in your test settings, e.g.
Or, if you don't overwrite the subject but, when using a champion challenger, set 100% of recipients to receive the challenger:
Note that this method doesn't apply to A/B testing because in this scenario all test variants would be sent to all of the audience instead of the typical all test variants sent to a portion of the audience then the "winner" being sent to the remaining.
I will note thought that personally I always set my subject line in the email to be a subject line I intend on testing. Following these methods invites a degree of risk that the test won't be added and an email will go out with the subject "A/B TEST THIS EMAIL". I would rather accidentally forget to A/B test than do that