Please refer to this thread, this should help: https://nation.marketo.com/thread/4348#13930
What is the difference between
Filter "Was added to list" and Trigger "Added to list"
Hi Nimisha,
Triggers are active, so anytime someone does a certain activity(click, register, etc) then something happens (your flow step).
Filters help to narrow down what you're looking for (ex: say you want to send email to to people who have their country is "USA", you will use filter here)
Similarly, Now "Was added to list", will be used if you want to send an email to the people who are already a member of a static list. And, " Added to list", trigger will send an email to all the people as soon as you add the people to the list. (It's just like action and reaction).
Hope, I am being clear here.