It is also known as a 'ghost' booking. The time frame that you must book it is normally about two years. If gives you time to look into future cruises without being pushed to decide while onboard your present sailing.
The onboard credits associated with it vary by the length of cruise and type of cabin you book when you decide. These shipboard credits can also be combined with any onboard credits that your travel agent might be offering on the cruise you choose.
You can either assign a Travel Agent to it when booking, or choose one later on. If it is not assigned, then you have to fax a letter to Princess authorizing the name of the Travel Agent that you want it transfered over to.
We have found it to be a great asset, and it sure helps to beat those Cruise Blues when you know that there will be another one booked soon.
It works very well also if you might be interested in a back to back cruise (you just go ahead and book two phantom (ghost) sailings), that way your deposit has already been put down on them and you don't have to come up with the rest until it is time for final payment. Much cheaper than the normal deposit that you have to pay.