Text, Numeric, Picklist, Text Label, Boolean and Date Elements can all be set to a default value, some of which can be set to read-only and some of which can have invisible values that are not viewable on the Smart Form UI but are visible on Salesforce for the Form Response. 


Please see this article for more information on the differences between 'Hidden' and 'Invisible' Elements: Difference between 'Hidden' and 'Invisible' Elements



Navigate to the Defaults Tab of the Edit Properties Window


To set a default value for any of the elements highlighted above, navigate to the Defaults tab of the Element Properties Window. In the Default Value text box enter either a default value or a formula. Three radio buttons will appear under the text box for Text, Numeric, Picklist, Boolean and Date Elements.


Text Label Elements are by default Read-Only and can only be Read Only. In this case when a default value is set for Text Labels, no radio buttons appear. If no default value is defined or if the formula returns a blank value, the normal Label defined under the General tab is used. 


When creating a Picklist in Manage picklists, there is a toggle option to use the first value as the default. The Defaults tab in the Edit window overrides this setting if a value is defined. For more on Picklist Elements, please see this article: Picklists - How to Configure a Standard Picklist



Mode
Function
Normal The default value will be populated in the form but is editable by the user.
Read-Only The default value will be populated but will not be available for editing. 
Invisible The field will be invisible on the Smart Form UI but will be available in the corresponding Form Response so mappings can be applied.


It is good practice to consider the configuration of the Visibility tab in the Edit window of an Element, as the Visibility tab has controlling factors that take precedence over the radio buttons in the Defaults tab. 


If you have a Visibility Formula that results in the Element being hidden, no information will be sent back to Salesforce, even if the Element is set to Invisible under the Defaults tab.


Save your changes.