If the purchaser fails to provide the written statement required by paragraph (a)(2)(ii) of this section, the purchaser is liable for the tax on the later taxable sale or use. If a purchaser acquires both fuel that is to be used for taxable purposes and fuel that is to be used for nontaxable purposes, and the fuel that is to be used for taxable purposes is stored in a different storage tank (or container) from the tank used to store the fuel to be used for nontaxable purposes, the written statement described in paragraph (a)(2)(ii) of this section will relate to the fuel to be used for taxable purposes if proper records are kept by the purchaser that sufficiently identify the tanks (or containers) into which tax-paid fuel is delivered and the quantities of fuel delivered into those tanks (or containers). If only occasional sales for delivery into a bulk storage tank (or other container) are made to a purchaser, a separate statement must be furnished for each order. However, if sales are regularly or frequently made to a purchaser, a written statement covering all orders for a specified period not to exceed 12 calendar quarters is acceptable.
26 C.F.R. §48.4041-5