Separating holdings from annotations makes great sense to me. For example, the book sitting on my shelf is a bf:HeldItem. I can literally hold it in my hand.
Is the bf:HeldMaterial superclass intended to extend that set to cover non-physical materials, like an e-book? (I’m willing to believe those are within the bf:HeldItems set too, depending on how the term is defined). Or perhaps bf:HeldMaterial is intended in a traditional sense of “holdings” where we know a specific library has 3 copies in their inventory, but those copies haven’t been clearly identified yet?