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This page exists to answer one question: when nothing sits between a reader and the ad server, does a creator-owned ad unit get decided at request time? It is the positive control for the surface matrix. Every hosted platform is measured against what happens here.
If the ad below fails to rotate on reload, the instrument is broken and no other row in the matrix can be trusted — including the Substack result. That is the point of running it.
The markup above is exactly the token published on every other surface: an image wrapped in a link, nothing more. No script, no iframe, no cache-busting query string. The image URL is requested from the rail on each load, and the rail chooses which creative to return at that moment.
Reload this page. The creative should change between loads. If it does, request-time decisioning survives end to end on a surface the creator controls, and each impression appears in the rail's log carrying this page as its referer.