The three-way minimum rule, which cities count as metro, and the documents that make your claim stick.
Under Section 10(13A) the exemption is the minimum of:
Basic ₹50,000/month, HRA ₹20,000, rent ₹25,000 in Mumbai. (1) ₹20,000; (2) ₹25,000 − ₹5,000 = ₹20,000; (3) 50% × ₹50,000 = ₹25,000. Exempt = ₹20,000 a month, ₹2.4 lakh a year — the whole HRA.
Same person in Pune paying ₹15,000: (2) = ₹10,000, so only ₹10,000 is exempt and ₹10,000 is taxable.
Rent receipts or a rent agreement, and the landlord's PAN if annual rent exceeds ₹1 lakh. Paying rent to a parent is allowed if they genuinely own the house and declare the income; rent to a spouse is usually challenged.
HRA exemption does not exist under the new regime. Compare regimes before choosing — at higher rents the old regime may win.