At which stage of supply VAT becomes payable?

In cases of taxable supply, VAT shall be payable in the event of any of the following activities whichever occurs first-

1.  In case of ordinary supply-
   • When supply is delivered.
   • When invoice will be issued.
   • When value of supply will be received in full or in part.
2. In case of progressive or periodic supply-
   • Separate invoice is issued against each.
   • When value of goods in part or in full will be received against each.
   • Value becomes payable.
   • When payable sum can be ascertained.