Conversion measurement reports have information collected in two different first party network contexts (publisher page, report destination (?), and are potentially reported to yet a third site. To avoid leaking information between different first party network contexts, the reports should therefore be specified to each use a single-use Network Partition Key.

In Chrome-land, these are known as "Transient NetworkIsolationKey" (NIK predates adding Network Partition Key to the fetch spec, so NIKs and NPKs are basically the same thing), but I don't believe the fetch defines them. They're just NPKs for opaque origins, which the fetch spec allows.