Rounding off Invoice

Taxable Value. 670.00/-
IGST 12% 80.40/-
Round off. (.40/-)

Invoice Value. 750.00/-

Portal accepts all values as input by us, except round off value i.e 40 paisa

Portal does not calculates Invoice value itself, else it would be 750.40/-

Question is...What is round off value?
Supply?
Book Entry?