The word, “assessment” used in the Income Tax Ordinance, 1984 means computation of the total income and determination of tax payable on such income. First all incomes shall for the purpose of charge of income tax and computation of total income, have to be classified and computed under the heads of income, namely, (a) salaries, (b) interest on securi¬ties, (c) income from house property, (d) agricultur¬al income, (e) income from business or profession, (f) capital gains and (g) income from other source under section 20. Income tax as one tax shall be payable on such total income. ….. (63 DLR (2011) 308)