The term "flag event" refers to a type of step that can be incorporated into your automation (a.k.a. drip campaign).
The Flag Event allows you to evaluate an action based on previous step. For example, evaluate if a included campaign was opened or a link was clicked and based on that information send the contact through a separate branch of the automation.
Flag Events, along with "Wait for Event", are the steps that allow your workflow to react to contact's behavior.
There are a lot of options to choose from with a Flag Event! Once you drag the Flag Event element over to the desired step in your workflow, click the pencil to select what action to flag and set specifications.
Opened: This flag event will evaluate if the contact opened a campaign you specify. The campaign MUST be included within the existing automation workflow (not another, unrelated campaign).
Clicked on Email link: This flag event will evaluate if the contact clicked on one or multiple links in a specific campaign. The campaign MUST be included within the existing automation workflow (not another, unrelated campaign).
Clicked on SMS link: This flag event evaluates if a contact clicked on a link in a previous SMS message from the same drip campaign.
Purchased from clickstream / eCommerce Store: You may note that these options are locked if your account does not have Clickstream or a Commerce integration enabled. If enabled, these flag events will evaluate a completed purchase.
Currently Subscribed to SMS: This flag event may also be locked. This step evaluates we have a SMS keyword attached to the email address/ subscriber.
Evaluate contact data: This flag event allows you to evaluate one or more database fields associated to a contact typically looking for a change. Functionally, this process uses clauses (equal to, contains, etc) so it is very similar to building segment caluses.
Evaluate category: This flag event evaluates if a contact is a member of a category or not.
Note: You can add additional nuance or complexity to certain Flag Events by adding multiple groups or values as seen here and below: "Can I Use Multiple Values in a Flag Event Step?"