An archive is another tasklist that is based on the current list. Completed tasks are moved there to remove them from the active list. Archiving helps to reduce the file size of active lists. This reduces clutter and improves file open/save time and other handling.

Archived tasks don't necessarily need to be removed from the active task list. Archiving and task removal are two separate operations which can be controlled independently. See below.

The archive file is a full ToDoList .tdl file, but only contains completed tasks. The file name is Name.done.tdl, where the source file would be Name.tdl. It is located in the same folder as the related active tasklist.

Completed tasks can be manually archived from menu > Tools > Archive Completed Tasks. This function moves completed tasks to an archive file, and optionally removes them from the source task list depending on your preferences.

The same function can be performed by selecting one or more tasks, right-click for the context-menu, then select Archive Selected Task(s). With this option, tasks that are not completed can/will also be archived.

Completed tasks can be auto-archived using Preferences > File Action Preferences with setting “Archive completed tasks”. This operation is performed when a tasklist is opened, not when the status changes - this way if tasks are removed, they don't disappear while you're working on them.

The operation to remove archived tasks is enabled from Preferences > File Action Preferences - check “Remove archived tasks from the tasklist”.

With a preference option under that “Unless they are flagged”, tasks which do not have the Flag field set will not be archived.

Internals : A .tdl file created as an archive contains an additional Archive attribute on the TODOLIST node.

You may open an archive to see history of completed tasks. The tab does show “(archive)”. But to avoid confusion between archives and a live tasklist, check the above preference “Display a warning if I attempt to add or delete tasks from an archive”.

You probably will not want a parent task to be removed on archiving if there are still sub-tasks which are not completed. For this, use the preference checkbox under “Remove archived tasks from the tasklist”, and set “Only when all the task's subtasks and siblings are also complete”.

There can be issues with archiving Reference Tasks.

Modification of an archive can lead to problems. For example, adding a new task into an archive will assign a task ID which may conflict with an ID in the original tasklist.