Custom fields are the org-defined dictionary of structured data your organization layers onto core resources — jobs, offers, openings, candidates, applications, and users. Each row defines one field: its field_type selects the parent resource it attaches to, and its value_type selects the data type and editor (text, number, currency, single/multi-select, date, user, attachment, etc.). Replaces the V1 path-scoped endpoints /v1/custom_fields/{field_type} — filter by field_type as a query parameter instead. For select-type fields, options live on the sibling resource /v3/custom_field_options; office/department visibility lives on /v3/custom_field_offices and /v3/custom_field_departments.