FBNW isn't preferred these days, the usual ones are:
Fit To Receive - all the work is done including wiring, software integration etc. Show up with the new system on a truck, bolt them in and you're ready to go. The RN does Phalanx this way and transfers the systems between ships as needed.
Installation Provision Made In Design - all the fittings and installation routes have been designed, but not actually put aboard. We can fit the new kit fairly quickly, depending largely on how complex it is, but it's a dockyard job.
Space and Weight Reserved - There's a big box on the drawings and a line in the weight database. Actually asking for it may involve the designers saying things like "How did we think this would work again?" Even figuring out how to do it might take years and tens to hundreds of millions....
As I understand it, FBNW is somewhere between IPMD and FTR - the seats and some services are in place, but it's still a dockyard job. Of course, the further down the scale you get the more variation in the kit can be accepted - if the design has an FTR Mark 1 Fulminator, but you decide to install a Mark 2 at midlife that's a metre longer and needs 50% more power, you're screwed. If it was only a space and weight margin, it can be sorted out in an afternoon with the customer, engineers and lots of coffee.
