Filter by user properties or applicable custom field values. For custom fields, add one extra top-level filter entry named customFields.<identifier> per criterion, for example filter[customFields.invoiceReference]=INV-42. Custom-field criteria combine with the user filters using AND.

Operator-scoped criteria match only users whose current operator owner is assigned to the field; retained values from inapplicable definitions do not match. Non-empty unknown, inaccessible, disabled, deleted or non-filterable custom-field keys return 400 naming the rejected keys; malformed values for usable fields return 422. Custom-field filters apply only to listings, not search.