void mongoc_bulk_operation_set_hint (const mongoc_bulk_operation_t *bulk, uint32_t server_id);
Specifies which server to use for the operation. This function has an effect only if called before mongoc_bulk_operation_execute.
(The function name includes the old term "hint" for the sake of backward compatibility, but we now call this number a "server id".)
Use mongoc_bulk_operation_set_hint only for building a language driver that wraps the C Driver. When writing applications in C, leave the server id unset and allow the driver to choose a suitable server for the bulk operation.