Short answer
Form 124, earlier Form 12BB, is the declaration an employee gives the employer of rent, home-loan interest and tax-saving investments, so TDS reflects them.
In AdviHR
Employees fill the declaration in AdviHR, including landlord and lender details, and payroll uses it for TDS.
Most of these deductions apply only under the old regime, so employees on the new regime usually have little to declare.
How sure are we? These figures use the same rules AdviHR payroll applies. We publish what has been checked against an official source and what has not, on the statutory accuracy page.
General information, not legal or tax advice. Confirm against the official notification and your adviser.