Per CSR metadata application profile:
Unit of extent required for dead serials, even if total extent unknown (300)3.4, LC-PCC PS
"LC practice/PCC practice for Core element: For serials and integrating resources, both the number of units and the term indicating the type of unit are required for a completed or "dead" resource (in any format) for which the total extent is known. Only the term indicating the type of unit is required for a currently-published tangible non-print resource (e.g., computer discs), for a currently-published integrating resource, or for a completed or "dead" resource (in any format) for which the total extent is not known. Extent is not required in all other cases."