What is the meaning of "open to sky"

Dear Sir,

I have bought a flat in five storeyed building. My flat was on third floor. There is a open area infront of my flat(at east side), and the place is mentioned as "Open to Sky" in my sale deed document.  Actually I paid money for that open area, but the details were not mentioned in sale deed. Now the builder is playing games with me.

Now my question is : What is "Open to Sky" means in sale deed documents? Shall I use it my own purpose?

NOTE : There is no building permissions to 3rd and 4th floors.