In short
An independent unit is a part of a building suitable for use on its own and capable of separate ownership. Apartments, shops, offices and separate storage rooms fall within this definition. The subject of a title deed is not the apartment but the independent unit.
When freehold ownership is established, every independent unit in the building is numbered and registered on the deed with that number. The number comes from the layout in the project and need not match the number on the door. Whatever number appears on the deed is the space you own.
Alongside the independent unit there is the concept of an appurtenance. Areas such as a parking space or a storage room are either defined as separate independent units or treated as appurtenances of one. The difference matters: an appurtenance cannot be sold on its own and transfers with the unit it belongs to.
Why it matters to an investor
This definition draws the boundary of what you are buying. Any mismatch between the apartment you saw and the independent unit you take title to surfaces at transfer, not at sale. Whether the parking space is an appurtenance or an independent unit must also be written clearly into the contract.