What is the extent of liability of instruments to stamp duty where several instruments are executed in a single transaction? Explain.

Section 4 of the Indian Stamp Act, 1899 provides that where, in the case of any sale, mortgage or settlement, several instruments are employed for completing the transaction, the principal instrument only shall be chargeable with the duty prescribed, for the conveyance, mortgage or settlement, and each of the other instruments shall be chargeable with a duty of one rupee instead of the duty (if any) prescribed. The parties may determine for themselves which of the instrument so employed shall, for the purposes of section 4(1), be deemed to be the principal instrument. Provided that the duty chargeable on the instrument so determined shall be the highest duty which would be chargeable in respect of any of the said instruments employed. However, notwithstanding anything contained in sub-sections (1) and (2), in the case of any issue, sale or transfer of securities, the instrument on which stamp-duty is chargeable under section 9A shall be the principal instrument for the purpose of this section and no stamp-duty shall be charged on any other instruments relating to any such transaction.