Add section on empty and absent values.
An incomplete submission form is a data form of the type "submit" that contains all required fields but some optional fields are omitted. The receiving entity of an incomplete submission form SHOULD only process (e.g., apply) the submitted fields. If applicable, the values of the omitted fields SHOULD keep their current value. The current value is often the value found in the corresponding form of the type "form".
As specified in the previous section, the values of the omitted fields SHOULD keep their current value. From this, it follows that 'unsetting' a field value requires a request to contain a <field/> element.
+This XEP does not standardize a way to differentiate between an "empty" and "absent" value. Unless other XEPs explicitly define these semantics, implementations can be expected to use a variety of representations, such as <field><value/></field> or <field/> to signal the absence of a (non-empty) value.
+Data validation is the responsibility of the form-processing entity (commonly a server, service, or bot) rather than the form-submitting entity (commonly a client controlled by a human user). This helps to meet the requirement for keeping client implementations simple. If the form-processing entity determines that the data provided is not valid, it SHOULD return a "Not Acceptable" error, optionally providing a textual explanation in the XMPP <text/> element or an application-specific child element that identifies the problem (see &xep0086; for information about mappings and formats).