PIO, stand from Persons of Indian Origin Card, is a little book residence permit issued by the Indian government for foreigners (passport outside India). As information mentions on the immigration stamp, the card holder free entry visa to India. So it can say that the book serves as a substitute for a visa.
In the booklet stated clearly some details, including holder's photo. For PIO which has indefinite expire date, was written "Long Life" on Date of Expiry column. For additional information written also that the card should be used with the valid passport.