This hook is available in Subscribe2 HTML only. It allows the email type of Public Subscribers to be changed. By default in Subscribe2 HTML the email type is full content HTML, in the free version of Subscribe2 is it a plain text excerpt. Using this hook the email type can be specified at site level.

function my_preferred_format() {
    // return one of the possible email types:
    // 'html_body' for a full content HTML email
    // 'html_excerpt_body' for an excerpt in HTML format
    // 'plain_body' for a full content plain text email
    // 'plain_excerpt_body' for an excerpt in plain text format
    return 'html_excerpt_body';
}
add_filter('s2_public_subscriber_format', 'my_preferred_format');