WP_List_Table::column_default( object|array $item, string $column_name )

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This is method that is used to render a column when no other specific method exists for that column. When WP_List_Tables attempts to render your columns (within single_row_columns()), it first checks for a column-specific method. If none exists, it defaults to this method instead. This method accepts two arguments, a single $item array and the $column_name (as a slug).

NOTICE: As of WordPress 3.5.1, in core $item is passed an Object, not an array.