When attempting to UNLOAD SEGMENT CATSYS, received error UT003018 SUBSCHEMA DEFINITION INCOMPLETE FOR RECORD LOADHDR
IDMS - all supported releases
The catalog segments actually need to be unloaded and reloaded by area, as the areas are in different subschemas.
The DDLCAT and DDLCATX areas must be unloaded and reloaded using the IDMSCATZ subschema and the DDLCATLOD area must be processed using subschema IDMSCATL.
This is documented in the UTILITIES manual in the UNLOAD Usage section "The Unload/Reload Subschemas".