I think you might be referring to deferred segment allocation ?

Think of it this way:

create table T ( x date ) tablespace users.

Normally, that would allocate (say) 10meg of space in the USERS tablespace. Now what if I never, ever use that table again? I've wasted 10meg.

Now you might be thinking - so why did I create it in the first place? Well, for many package applications, they often install every table that *might* be used by customers, only to have 100's or 1000's of them remain empty forever.

Deferred segment allocation simply says "We wont allocate that 10meg of space until someone inserts the first row"

Hope this helps.