I had thought that ISO 9000 defined special processes but I have not been able to find it today. Although it is a different standard, AS9100 does mention special processes. In AS9100B and AS9100C it was in an aerospace enhancement note to 7.5.2 simply stating that these processes (the ones that cannot be verified by ordinary means) were also known as special processes.
In AS9100D the note remains where they are first addressed in 8.5.1f. 8.5.1f also directs the reader to 8.5.1.2 where essentially aerospace restored the wording from ISO 9001:2008 and AS9100B/C. ISO 9001 had streamlined the requirement, mainly removing the requirement to keep records. AS9100D brought it all back. 8.5.1.2 reinforces the topic by titling it "Validation and Control of Special Processes".
Core to both the older wording and what is not in AS9100D 8.5.1.2 is "For processes where the resulting output cannot be verified by subsequent monitoring or measurement, the organization shall establish arrangements for these processes . . . "
In brief, if you can't verify it, you need to validate it.
Although it is not the standard you are dealing with, you may find more insight searching in the AS9100 literature.