What is "Registration Code" in Marketo?

Hi there - 

I noticed in my Marketo instance that if I look at the Members tab for a campaign, one of the fields I can add to the view is "Registration code". I added that to my view and unique codes populated in that field for every member of the campaign. How is this code being populated? Can I control what goes in there? We don't have a UTM parameter for registration code or anything that I'm aware of that we doing to populate this field, yet it appears to have data in it for every person. Thank you.