Action targets

The actions target is on who the actions will be applied, like the conditions this steps will change according to the kind triggers you've chose.

If you chose a classic trigger then you can execute action on all the users or just a part, depend on the filters you've chose. Otherwise if you've chose a trigger based on a user action you'll be able to execute action for all the user and just the who triggered the automation.

If you choose to execute the actions on the user triggering the automation
If you choose to execute the actions on the a part of your user with a filtre

Note that if you want to execute your actions on all your users, you can set 0 filter and save, this way AcyMailing will execute the automation on all users.