SDK 15.3.0 documentation says:
"Error codes are 32-bit unsigned integers with the most significant 16-bit reserved for identifying the module where the error occurred ... A range is reserved for application as well, it can use this range for defining application specific errors"
infocenter.nordicsemi.com/.../group__sdk__error.html
(my emphasis)
The same appears in the SDK 16.0.0 documentation:
https://infocenter.nordicsemi.com/topic/sdk_nrf5_v16.0.0/group__sdk__error.html
But nowhere in the documentation or the source code can I see any definition of the range for Application errors!
So what is it?
This has come up several times before; eg,
https://devzone.nordicsemi.com/f/nordic-q-a/16283/error-codes