Check a salary-arrears relief figure under section 89(1). Recomputes Rule 21A across both years and says whether payroll used no relief, or a different regime in one.
Why does this page ask for taxable income, not gross salary?
Because Rule 21A compares tax on total income. The pay structure, HRA and deductions have already happened by the time a Form 10E or a payroll relief figure exists. Using taxable income keeps this page on the rule it is actually checking.
What if the arrears belong to more than one earlier year?
Then this page is too small on purpose. Rule 21A spreads the arrears back year by year and totals the tax there. This page checks the common single-year case only - one arrears amount, one due year - and says so out loud rather than pretending one year can stand in for several.
Can payroll use a different regime in the year the arrears belonged to?
Yes. The relief is priced on the tax actually applicable in each year, and the chosen regime can differ across years. That is why this page asks for a regime twice and tests the opposite regime as one of its explanations when payroll's figure disagrees.
Does this page tell me whether I must file Form 10E?
It does not tell you whether you must. It tells you what Form 10E is for: furnishing the particulars of salary paid in arrears or advance when relief under section 89(1) is claimed. That wording comes from the Income Tax Department's own Form 10E help entry, and the page stops there.
What if the rule figure is zero and payroll gave zero too?
Then the arrears were not taxed any harder in the year paid than they would have been in the year they belonged to, on the two taxable incomes you entered. Section 89(1) is a relief from bunching into a higher rate. With no bunching, there is nothing to relieve.