Remapping is really just the modern name for chip tuning , you used to have to physically remove the memory chip that held the maps before you were able to modify them, then solder or push it, or a completely new chip , back in.. from about 2 years ago "remote" devices became available that could read the chip via the diagnostic port..
On transits the exception to this general rule of progress is the lucas epic injection system used on mk5's . These use a one time programmable chip that you remove , read & discard.. a new "many time programmable" ( dig the grammar
) chip is written with the modified data then put back in the ECU. The processor of the ECU still wont allow for remote programming despite the new chip, so a remap on these is a much longer & more involved process, especially if tweaks are needed, as you have to repeat the whole process.. This is how all "chip tuning " was carried out from 80's Cosworths right up to when the current remote devices were introduced & the length & complexity of the process was the were the main reasons why the cost was often £500+ & why £250 quid tuning boxes became so popular ...
Most tuners now use remote devices to read & write the chip in the ECU that holds the maps & they use the chip removal method as a backup or on very latest vehicles of which the remote devices have yet to cover..
Theres another method of reading & writing an ECU's memory chip called BDM..( bench diagnostic mode) this involves removing the ECU & connecting to some pins on the circuit board of the ECU.. After assembly of an new ECU, the factory would use this method to program the ECU.. The benefit of this method, from a tuners point of view is that it bypasses the need to solder, it bypasses some security processes that attempt to keep tuners away from the copyrighted data & bypasses the possibility of errors that arise from remote programming procedures. Its relavent because the ECU's can easily be damaged or written off just from a programming error & BDM offers the least risk of this & can in some cases recover ECU's " damaged " by other methods..