Custom field options are the selectable values on a single_select or multi_select custom field — each row belongs to exactly one custom field via custom_field_id. Filter to a specific dictionary with custom_field_ids (preferred when the id is known) or custom_field_key (e.g. school_name, degree, discipline, cost_center). The V1 endpoints /v1/schools, /v1/degrees, and /v1/disciplines are all served here in V3 — pass the matching custom_field_key. Archived options remain in results by default; pass active=true to restrict to currently selectable options.