What does Marketo use to consider what is included for multi-touch attribution?

I fully understand the concept of multi-touch attribution. This question is in the detail of when an activity is considered attributed? Is it any touch when they are sent or visit something, or is it Success for a particular program? I read the term "conversion" but that means many things to many people. What does the Marketo system use to include an action as part of multi-touch attribution? Thanks.