What is the Difference between "Filter" and "On Action" ?
On actions – trigger the next element as soon as an action was made while filter allows you to check an action over time. When selecting "on action" option the contact made an action on the previous element he will immediately move to the next element. You can trigger the next element based on email open click, page view visit and more. Unlike filtering, "On action" cannot trigger negative actions (for example, contacts who didn't open the email.
For example: If you use two filters to check if a user has opened / didn't open your email and connect between them using "on Send" trigger like this:
The first email will be sent and the contact will immediately move to the "hasn't opened" filter. This is why the right way to use filters is by adding "delay element" that will check if a contact has opened the email after 2 hours. Like this: