It rather depends if you think the settings are likely to be changed by an incompetent person; It is possible to seal the settings on some designs, either with wire seals or paper ones, and changing the settings is then a deliberate act, perhaps requiring tools- after all in that case, in a conventional CU, someone could unscrew a fixed rating say 32A MCB and fit  40A one in its place without much more effort.

You can only base your design on the settings you sensibly expect  to be used, and the rest is a matter of clear labeling and instructions.

Mike

Is it permissible to apply the Zs value for the actual settings, despite the fact that these could be increased at a later date.

That's always been a risk - rewireable fuses could have the wrong fuse wire fitted, many cartridge fuses of different ratings are the same physical size (e.g. a 20A BS 1361 is the same size as a 15A one, and of course 13A fuses may be replaced by 13A in FCUs and the like). Correct labels are considered sufficient.

I agree BS 7671 doesn't spell that out for ADS, but for overcurrent protection it does make it clear that the "rated current or current setting of the protective device" (my emphasis) is what counts (433.1.1) so that might also be reassuring if you expect consistency.

    - Andy.

The whole point of the adjustability is to allow the device to be matched to both the characteristics of the supply and the load. Any adjustment will be deliberate in a properly controlled system and the person doing the adjusting will need to be competent to do so.