...

If the user changes his/her mind and removes the value from the Street field,  will recalculate the validity of the form and infer that the Address section is no longer invalid since it is optional. The generated XML instance document will also not contain an address element. Once again,  is automatically ensuring that it is not possible to submit a form that is in an invalid state and that would generate an invalid document.

You can find information about the section Required property here.

Input Validation

Form fields added from ' control palette have built-in validation rules.  The table below details the default validation for each control type in ' palette. Patterns added to a control change the defaults.

...