Burden of proof in interstate goods transfers requires registers and original declarations to substantiate non-liability to tax. Rule 9 requires a transferor claiming non-liability to tax for goods sent out of Delhi to maintain a true and complete register in Form 7, produce it and related particulars to the Commissioner, and furnish the original portion of the declaration Form along with the reconciliation return; the Commissioner may also demand additional documents and evidence to verify the genuineness of the claim, including specified records when goods are transferred to an agent in another State.