Footprint: where a product is listed, over time
A product's footprint is the set of directory listings we have observed for it: which tracked directories link to its domain, whether each link is still there, whether it is followed, and whether the page carrying it is indexed. It is the object everything on this site reads and writes.
What one listing records
A listing is an observed edge between a product domain and a directory: the page we found it on, the link's rel attribute, the date we first saw it and the date we last saw it. When an attribute changes or the listing disappears, the record is closed and a new one opens. Nothing is overwritten, so the footprint carries its own history.
Where footprint data comes from
Where to see it: every product's footprint is on its page in the product index, a scan shows it for any domain you type, and the portfolio shows it across every domain you claimed.