Free text a publisher adds to its own tracking link, such as a page or a placement. It describes a click, not who ran an ad.
A sub ID is a field on a publisher's tracking link that the publisher fills in itself. Neither the brand nor the network sets it. It might hold the page the link sat on, a placement such as “Header banner, page 2”, a campaign name, or an internal code for one of the publisher's own members. On the link it's a query parameter, such as subid=homepage-banner, under whatever name the network uses.
It travels with the click to the sale, so it can tell you which of a publisher's pages, or which of a sub-network's members, a sale came from, if you're shown it. Networks differ: on some, a link can carry several and the brand sees only the first; on others, the brand sees none of them.
- It doesn't name a Google ad, a keyword or the Google Ads account that paid for a click.
- A publisher running paid search can fill it from ValueTrack parameters in its tracking template, so it may hold a keyword or a campaign id.
- Ask your network which field reaches your reports, and who can match a code in it to a publisher and a domain.
Behind the network shows where sub IDs help and where they stop.
Google's own help and policy pages first.