If the same document can be categorized under two types of document subject to the same tax rate, only tax stamps applicable to one type of document are required; if the same document is subject to different tax rates, the higher rate shall govern. 
Where a document subject to a higher tax rate is substituted by a document subject to a lower tax rate, the document shall be affixed with tax stamps in the amount calculated at the higher tax rate. 
Where a taxable document is substituted by a tax-exempt document, tax stamps corresponding to the type of the taxable document shall be affixed.