If you would like your followup emails to send to all contacts that were sent the parent tactic, select Did > Become a Tactic Member > blank. Leave the text field blank if you are utilizing the parent tactic indicated. You also have the option to search the parent tactic name and include it in the blank space, but this is not required because it's already indicated in the parent tactic field. You can also select other tactics if needed by typing in the tactic name and choosing from the drop down, but usually you will be utilizing the parent tactic only in a drip campaign.
If you use the Lifecycle, all stages are checked by default when creating a followup because segment criteria and the Event selected determines who the email is sent to. However, the parent tactic could have sent to contacts in all stages, for example, but you only want the followup to send to those in the Advance stage. If you only want the followup to send to a specific stage, only check the stages that apply.