What it is
The sub-registrar office, often shortened to SRO, is where a sale deed or other transfer document is presented, verified and registered under the Registration Act. Tamil Nadu is divided into SRO jurisdictions, each covering a defined set of revenue villages, and a document must be registered at the SRO covering the property's location, not any office of convenience. The sub-registrar checks the document, collects stamp duty and registration fee, and enters it into the office's records before returning a registered copy.
Why it matters when you buy
Registration at the correct SRO is what makes a sale deed legally effective and gives it evidentiary value; an unregistered or wrongly registered transfer document offers far weaker protection. The SRO is also where past transactions on a property are recorded, which is why an encumbrance certificate, a summary of registered transactions, is issued for a given survey number and period, and is one of the first things a buyer should check before paying.
Where you find it
You can look up which SRO covers a given revenue village on tnreginet.gov.in, which also lets you apply for an encumbrance certificate and check the guideline value for that jurisdiction online, rather than visiting in person for those specific services. Sub-registrar offices themselves remain the place to physically present and register a sale deed.