WP_List_Table::get_sortable_columns(): array<string,

The format is: <ul> <li>'internal-name' =&gt; 'orderby'</li> <li>'internal-name' =&gt; array( 'orderby', bool, 'abbr', 'orderby-text', 'initially-sorted-column-order' ) -</li> <li>'internal-name' =&gt; array( 'orderby', 'asc' ) - The second element sets the initial sorting order.</li> <li>'internal-name' =&gt; array( 'orderby', true ) - The second element makes the initial order descending.</li> </ul> In the second format, passing true as second parameter will make the initial sorting order be descending. Following parameters add a short column name to be used as 'abbr' attribute, a translatable string for the current sorting, and the initial order for the initial sorted column, 'asc' or 'desc' (default: false).