Z-Code is mentioned in the IBM manual SY31-0581-0
IBM 5110 Language Support Maintenance Information Manual
Towards the end of the manual, it lists a cross-reference of values between EBCDIC, Atomic Vector, Z Code, and ALC. EBCDIC is widely familiar, but the purpose and usage of the remaining categories of values is still obscure.
Z-CODE is also mentioned in the IBM 5100 MIM: