The $mod variable is defined early in the WPSSO Core load process and is passed to most methods and filters. The $mod variable name is short for "module" and it defines essential reference values for different WordPress and WPSSO object types:

An example $mod array for a post:

Array (
    [id] => 2717
    [name] => post
    [name_transl] => post
    [obj] => object WpssoPost
    [query_vars] => Array ()
    [is_404] => false
    [is_archive] => false
    [is_comment] => false
    [is_date] => false
    [is_day] => false
    [is_home] => false
    [is_home_page] => false
    [is_home_posts] => false
    [is_month] => false
    [is_post] => true
    [is_post_type_archive] => false
    [is_public] => false
    [is_search] => false
    [is_term] => false
    [is_user] => false
    [is_year] => false
    [use_post] => false
    [post_slug] => the-post-slug
    [post_type] => post
    [post_type_label] => Post
    [post_mime] => ''
    [post_status] => publish
    [post_author] => 123
    [post_coauthors] => Array ()
    [post_time] => 2013-03-15T22:23:27+00:00
    [post_modified_time] => 2021-01-31T00:16:46+00:00
    [tax_slug] => ''
    [tax_label] => false
)

An example to retrieve custom post meta:

if ( $mod[ 'is_post' ] ) {

    $value = get_post_meta( $mod[ 'id' ], '_example_meta_name', $single = true );
}

The 'obj' element can be used to call object methods. Here's an example to get a custom Open Graph description value (if one has been defined):

$og_desc = $mod[ 'obj' ]->get_options( $mod[ 'id' ], 'og_desc' );