I'm sorry if 'dumb' was a bit strong, but I do think you are choosing to misunderstand.
Perhaps it was the 'way' I read it but it didn't make sense to me....and I dont know what 'entrench' means or what a spotty kid has to do with it,anyway here goes.............
You have a immobiliser number, a key number & a remote number (some of these might not apply depending on vehicle/age/variant)
immobiliser number - this is the number needed to programme your keys into the body computer.This can only be done by examiner & must be carried out when a new key is purchased.
remote number - this is the number required to programme the remote part of the fob into the locking/alarm system again this can only be done with examiner,however mainly applies to Alfas.
Some remotes on fiats work straight away when purchased but most require the above immobiliser recognition sequence of keys to be carried out first before the examiner operator can carry out the remote learn sequence. This is why the garage requests you bring in all available keys because any not present at the time are wiped from the memory & become unusable.
key number - this relates to the cut of the key.
The factory holds all the above codes for reference/security. The ISR holds the above info in the UK (and
radio code if applicable) it is from this orginisation that the dealer gets the information to programme new keys. However if the customer has previously ordered a key card (with immobiliser number on it) and has it with them at time of going to garage to get a new key programmed then the dealer does not have to contact ISR thus saving time & in my opinion should get key programming carried out cheaper.
So what if the ISR does not hold the code(s)? This may be because the car is a import, not registered with the IRS, in this instance the dealer has to order a code card direct from the factory.
What if the locks,body computer have been changed? Unless the ISR have been informed of this & you require a new key then the dealer cant programme new key(s) to immobiliser as the codes ISR will supply will be wrong.
If I have missed anything say so.