Every case is unique, of course, and the reason for the Service Team to decide to replace the disc in one case might not be the same as their reason for so deciding in another case.
In general, though, what it comes down to is that if a given disc has a level of damage (or contamination that cannot be cleaned) such that the sensor will no longer be able to reliably measure with the specified accuracy, that would be a reason to replace the disc.
It should be noted that not every “marking” on a disc caused by laser exposure is necessarily “damage”. Sometimes it might be merely cosmetic. We generally define “damage” as a marking that changes the reading by 1% or more. If a given beam incident ON the marking reads more than 1% differently than the same beam NEXT TO the marking, then we consider that marking as damage.
In some cases, if there is a damage marking but it is small, perhaps near the outer edge of the aperture of the sensor where most of the time the beam will not actually be incident on it, a very experienced customer (perhaps an R&D physics team for example) might simply tell us to leave it as is, without repairing it, and they understand that they should avoid measuring on that one spot. For a sensor used in a Production environment, on the other hand, that would of course not be an option.
Again, every case is different, but hopefully the general idea is clear.
At the end of the day, we have to make sure we are providing our customers with a measurement instrument that will measure with the very highest possible accuracy.