How it is built
You create it in Meta's Audiences tool from your pixel as the source, then choose an event or URL rule: all visitors, or "People who visited specific web pages," which accepts URL matching so you can target a path or a "URL contains" pattern (handy for splitting one page by utm_campaign). You set a retention window in days (up to 180) and can layer up to five include or exclude rules. One catch: it does not backfill, it only populates from the moment you create it, so set it up before the spend starts.
Why it segments by page, not campaign
The audience is defined by the URL and the pixel events, never by the campaign or channel that referred the visit. That information is invisible to it. So two ad campaigns pointing at the same page produce one indistinguishable pool, and the only way to split retargeting by intent is to split it by URL: a dedicated page per intent, or a UTM parameter your rule can read. Seed a lookalike from a converter audience, not from all visitors. The full playbook is in why your retargeting is built from the page, not the intent.