When the done items section of a category itself starts to become
cluttered, or if you just want to store some accomplished todo items in
a separate file, you can move them to a Todo archive. This is a file
with exactly the same structure as a todo file, i.e., divided into
categories, but differs in that the categories contain only done items.
Todo archives reside, like todo files, in todo-directory but have
the extension ‘.toda’ instead of ‘.todo’.