New Delhi, March 17: The government today cleared the air on Aadhaar cards, saying they were not proof of citizenship and enrolment was "voluntary".

"It (Aadhaar) is not a proof of citizenship. Enrolment for Aadhaar is voluntary," minister of state for home Haribhai Chaudhary said in a written reply in the Lok Sabha. "It is for the implementing authorities to decide on the applicability of Aadhaar for delivering various services to the people."

The clarification came a day after the Supreme Court warned states against making the cards mandatory for services or social benefits.

The UPA government's direct benefit scheme was based on Aadhaar and had raised questions whether only cardholders could have access to essentials like LPG cylinders. The Aadhaar number, a unique identification number, is generated by the Unique Identification Authority of India (UIDAI). The cards are issued by the authority.

While enrolment for Aadhaar is voluntary, what Chaudhary has not made clear is whether acquiring an Aadhaar number is mandatory. This is because it is mandatory for any "usual resident" of India to get registered with the National Population Register (NPR). When one registers with the NPR, an Aadhaar number is issued.

"It is mandatory for every usual resident of India to register in the NPR," says the website of the Registrar General of India. In other words, even if someone has got an Aadhaar card, he will still have to get himself registered with the NPR, a more comprehensive database. In both cases, the resident receives a unique identification number generated by the UIDAI.

A usual resident is someone who has lived in an area for six months or more or intends to reside there for the next six months or more.