The description of Manifestations in FRBR-ER as being physical is generally considered to be an error - they are more readily understood as being sets of Items that are sufficiently similar (the Items are physical, but the Manifestation is abstract).

It is possible to to treat manifestations as being physical if one treats the items as being non contiguous parts of the Manifestation.  This is metaphysically coherent, but requires one to accept that pulping an unsold copy of the final printing of  "The Airport Book Conundrum" has changed the mass of that Manifestation, or that the position of a Manifestation for an ebook I have changes when I download it to my smartphone.

It is simpler to treat everything above the level of Item as being abstract, and that is the stance taken in FRBRoo.