cant say i know what im looking for when someone said about the arch extension.
Well having a look at your other thread, the chap kinda explains it. But basically on the LCR and cupra you have 'sportier' side skirts. These drop down to a certain level, and are rounded.
For the facelift cupra, the lowest part of the bumper in front of each wheel also is rounded, so that it looks the same as the side skirt. I'm guessing that the standard wheel arch/lining must be extended so that it the lower bumper is allowed to do this. I'm no expert, but thats what the description seemed to state. If you werent sure, then you should have just posted a reply saying so - somewill will end up helping, but posting multiple threads probably isnt a good idea really
But hope you can get it sorted in the end. Never really liked the facelift cupra bumper, but since I bought my LCR, strangely Ive grown to like it...