On the <em>Manage fields</em> page for your entity type or sub-type, you can add, configure, and delete fields for that entity type or sub-type. Each field has a <em>machine name</em>, which is used internally to identify the field and must be unique across an entity type; once a field is created, you cannot change the machine name. Most fields have two types of settings. The field-level settings depend on the field type, and affect how the data in the field is stored. Once they are set, they can no longer be changed; examples include how many data values are allowed for the field and where files are stored. The sub-type-level settings are specific to each entity sub-type the field is used on, and they can be changed later; examples include the field label, help text, default value, and whether the field is required or not. You can return to these settings by choosing the <em>Edit</em> link for the field from the <em>Manage fields</em> page.