When an instance document is validated against a schema (a process known as "assessment"), the schema to be used for validation can either be supplied as a parameter to the validation engine, or it can be referenced directly from the instance document using two special attributes, codice_1 and codice_2. (The latter mechanism requires the client invoking validation to trust the document sufficiently to know that it is being validated against the correct schema. "xsi" is the conventional prefix for the namespace "http://www.w3.org/2001/XMLSchema-instance".)