What field dose the "Source" Constraint map too in the "Add to List" Trigger?

[FerrisA]

Hi everyone, I am trying to identify what field the "Source" constraint maps to when using it on the "Add to List" trigger. However, whenever i add the constraint and select the drop down I get no results. So I'm not really sure how to identify this. 

[Darshil_Shah1 · Accepted solution]

Well, the source constraint as the name suggests what the source of activity was (in this case add to list activity), e.g., List Import, Marketo Flow Action, Web Service API etc. Also, FYR - source is a Marketo system managed field (it isn't mapped to any lead database field).