You have certain amounts of points( Starpoints or something like that they call it , always changing), and you use them whenever and wherever you want to.
For example , I need two one bedroom or 3 bedroom lockoff in Orlando in October , but because they have no available villa in our season at Harbour Lake , we probably will lose this year usage , because I was late with reservation. Probably they have av unit at another Marriott resort in Orlando , but we can't use it , only through II , and for a fee.
On the other side , I will try to book our week( actually just 4 days) at Vistana Villages , if there is no available unit , then just go to Vistana Resort. No fee , no II involved....