What it is
When you buy an apartment, you don't get a separately surveyed plot of land under just your unit — you get an undivided share, or UDS, in the whole plot the building stands on, held jointly with every other unit owner. Your UDS is usually expressed as a fraction or a specific extent, in square feet or cents, proportional to your unit's size relative to the whole project. It's recorded in your sale deed and forms the basis for your share of the common land, not a separately demarcated piece of ground.
Why it matters when you buy
UDS is what actually gives you a legal stake in the land, since the building itself depreciates while the land underneath it doesn't. Buyers should check that the UDS figure in the sale deed and patta documents matches what was promised, and that the sum of every unit's UDS in the project adds up to the full plot — developers sometimes over-allot or under-allot shares. A mismatched or missing UDS entry can complicate reselling the flat or getting patta transferred to your name later.
Where you find it
Your UDS extent is stated in the sale deed for your apartment, alongside your unit description, and it should also be reflected wherever the underlying land's patta-chitta is recorded, since the land patta is typically held jointly by all unit owners in the stated shares. When in doubt, cross-check the UDS figure across the sale deed, the encumbrance certificate, and the society or association's records for the project.