Child elements within the <Fields> element may use the Default attribute to assign values to fields when new records are created.

<Category Id="Category" Default="Blog Posts" Invisible="true" /> 
This attribute is often used along with Invisible="true" and with <Filter> tags to configure Views that only display a single type of Content or Contacts.

For example, the following tags in the View News uses a filter to only show existing records where the Value: "News" is included in the Category field.  New records set the Category field to "News" and hide Category from view - creating a View completely dedicated to News content.
<View Id="News" TableId="Content">
        <Category Id="Category" CatType="Content Types" Required="true" Multiple="true" Invisible="true" Default="News"/>

        <Filter Type="Required">
            <Eq FieldId="Category.Label" Value="News"/>