First let me start by saying there are more than one type of remote for the ford range of vehicles, the early models use a 2 button remote, of which there are 2, infra red (IR) and radio frequency (RF), it is easy to spot which is which when you remove the horse shoe part of the key.
The infra red (IR) key has a small bulb opposite the end of the red chip for the pats system, and is labelled 95 BG 15K601 AA,
While the radio frequency type, doesn't have the small bulb opposite the red chip for the pats system, and is labelled 97 BG 15K601 AA
Now we move onto the 3 button type which is most commonly used on later vehilcles (99 - on), there are inumerable different types of these keys, some have the red chip in them, some have the later light blue or dark blue chips in them, it all goes towards the part number, again you will have to remove the horse shoe part of the key, and with the aid of a magnifying glass you need to make a note of the part number, which is located next to the end of the glass chip in the remote.
The biggest majority of these numbers start 98 BG 15K601 AA, (Though some now start ST ***) now this is the important part it is these last 2 letters (AA), that changes according to the chip fitted, so be sure that the one you are wanting corresponds to the one(s) you have E.G. AA, AB, AC, and so on.
You may think it makes no difference to the remote, it doesn't, the remote will still be able to be synced to your vehicle to open the doors ect, but it does matter if you intend to use the key to start it, as with an incorrect matching of the chip, it cannot be programmed to your car.
One final note, from my own personal experience, I recently bought a 2000 mondeo, after having my 98 mondeo wrecked, and found that my 2 button remote (97BG 15K601 AA) from my old car, could be programmed to not only open, (Remotely) but start my 2000 Mondeo (3 button 98 BG 15K601 AA), thereby saving me some cash, (I only had to replace the blade and have it cut)