Your net chargeable income, i.e. assessable income after deductions and allowances, is charged at progressive rates. But if what you need to pay on the basis of your net chargeable income exceeds the tax charged at standard rate on your net total income, i.e. assessable income after deductions but before allowances, then you have to pay the lower amount of tax.
Assessment year refers to April 1 to March 31 of the following year.
Tax payable is computed on the net chargeable income an individual from April 1 of the previous year up to March 31 of the current year.
You may refer to the following link for more information on the tax rates and computation of salaries tax:
https://www.ird.gov.hk/chi/pdf/pam61c.pdf