Good debate this one, and a question I am asked daily is 'can you do custom maps and how is it better than a generic map?'
The explanation could go into 3+ pages but a general rule is that a generic map is a 1 size fits all product, suitable for a wide range of applications and is not and can not be adjusted by the end user. i.e bluefin or 'flash remaps'. Flash remaps are a simple procedure where you take the car to a 'tuner' ( I use the term tuner very loosely for this type of remap option) and they get an ID of your ECU type and software version and email it to a 3rd party that will copy and paste the changed settings that were made for a similar ECU and car into a file and send it back, this file is then flashed over your current software, you get the keys back and the 'tuner' hopes it doesnt have any problems. The worst type of flash tuning is usually carried out from a back of a van.
A custom map will be very different, the tuner will likely be making the software themselfes rather than sending it away to a 3rd party and the use of a dyno is essential for back to back testing and measuring, however the use of a dyno does not define a custom map, its just a very very handy tool to have to determine wether ther changes you have made are making any real difference to power, emmissions, torque and various other parameters. A dyno allows you to back to back test the effect of the customising of the map in a controlled and safe environment. I.e if you add ignition advance is it making extra power? Is that extra bit of fuel you have added at 3000 rpm at 80% throttle lowered the exhaust gas temps to a safer levels, or has it effected the power output in any way? You get the idea, its time consuming and costly but you get the best end product available, not essesntial for the average stage 1 car but for motorsport or highly modded stuff its a must.
Then you get the middle ground such as REVO, which offers the best of both worlds. A simple sliding scale to adjust the boost, timing and fueling properties within the map and can be accurately adjusted with on-road datalogging to get a good end product. Yes it is custom to a certain extent but lacks the resolution of a genuine custom map as you can only adjust 3 parameters on a 2D sliding scale, no single adjustments for fuel vs throttle position for example, but for a quick easy product they are the market leaders for a very good reason.
As a guide, anyone that offers a 'full custom map' without the use of a dyno is cutting corners a little. Anyone that offers a custom map without any datalogging is either missinformed or pulling the wool over your eyes. We had a customer in yesterday that had a 'proper custom remap' (his words) that cost just £200 and was finished in just 10 mins, he was enquiring about rolling road testing as he couldnt tell the difference after it was done apart from it was jerky at low throttle, the 'tuner' said it will get better when the weather isnt so cold