I discovered this while putting together a table of parameters for ArctosR documentation. This is referencing the catalog endpoint rather than the new record endpoint.
At https://arctos.database.museum/component/api/v2/catalog.cfc?method=about
14
display "Kingdom"
obj_name "kingdom"
query_cost 1
category "record"
description "Kingdom as provided in collection's preferred Source(s)."
default_order 150
15
display "Phylum"
obj_name "phylum"
query_cost 1
category "classification"
description "Phylum as provided in collection's preferred Source(s)."
default_order 160
The "kingdom" result parameter in the API documentation has its category set to "record" when its location in the list of parameters and its description would seem to me to imply that it should be a classification.