What it is
Poramboke describes land that isn't assigned to a private owner and isn't assessed for land revenue — village roads, tank beds, canals, grazing grounds, burial grounds, and other common or government land. It appears as a classification in village land records alongside nanjai and punjai, marking that a parcel is set aside for public or government use rather than private cultivation or habitation. Poramboke land is not ordinarily something a private individual can hold patta over or sell, since it isn't private property to begin with.
Why it matters when you buy
Poramboke matters to a buyer mainly as a red flag: if a plot you're looking at is recorded as poramboke, or if it borders poramboke land whose boundary is unclear, that's a reason to slow down and verify rather than proceed. Encroachment on poramboke land is a known problem in Tamil Nadu, and a patta obtained over land that is actually poramboke can be defective. Always confirm a plot's classification against the current patta-chitta and A-register before treating a seller's claim of clear title as settled.
Where you find it
A parcel's poramboke status shows up in the village A-register and in the patta-chitta system, which will simply not carry a private patta for land recorded as poramboke. The FMB sketch can help you see a plot's boundaries relative to adjoining poramboke land such as a road or waterbody. If anything about a boundary looks inconsistent with what's on paper, that discrepancy is worth raising with the revenue authorities before you go further.