"Fields 690-699 are reserved for local subject use and local definition. For interchange purposes, documentation of the structure of the 69X fields and input conventions should be provided to exchange partners by the organization initiating the exchange."
Vega does use these subfields for 6XX fields when defined in MARC.
MARC does not define subfields for 69X fields:
69X - Local Subject Access Fields (R)
https://www.loc.gov/marc/bibliographic/bd69x.html
"Fields 690-699 are reserved for local subject use and local definition. For interchange purposes, documentation of the structure of the 69X fields and input conventions should be provided to exchange partners by the organization initiating the exchange."
Vega does use these subfields for 6XX fields when defined in MARC.