As I explained there in detail, it all looks like there is some missing piece somewhere, something that the spec designers had in their mind, but haven't written down. And that's symptomatic: The spec contains multiple references to "other documents" and "elsewhere", but leaves implementors alone finding these. Even on a mere technical level, that's all hugely problematic. It looks like an unfinished early draft, but at the same time BIMI is already implemented and sold.