17 CFR 1.42: Delivery accounts.

In connection with the making or taking of delivery of a commodity under a commodity contract whose terms require settlement via physical delivery, if a futures commission merchant facilitates or effects the transfer of the physical delivery property and payment therefor on behalf of the customer, and does so outside the futures account, foreign futures account or cleared swaps account in which the commodity contract was held, the futures commission merchant must do so in a delivery account, provided, however, that when the commodity subject to delivery is a security, a futures commission merchant may, consistent with any applicable regulatory requirements, do so in a securities account.