Whether or not all activities will be removed from the task, except for the root activity, whenever it is re-launched from the home screen — "true" if the task is always stripped down to its root activity, and "false" if not. The default value is "false". This attribute is meaningful only for activities that start a new task (the root activity); it's ignored for all other activities in the task.
When the value is "true", every time users start the task again, they are brought to its root activity regardless of what they were last doing in the task and regardless of whether they used the Back orHome button to leave it. When the value is "false", the task may be cleared of activities in some situations (see the alwaysRetainTaskState attribute), but not always.
Suppose, for example, that someone launches activity P from the home screen, and from there goes to activity Q. The user next presses Home, and then returns to activity P. Normally, the user would see activity Q, since that is what they were last doing in P's task. However, if P set this flag to "true", all of the activities on top of it (Q in this case) were removed when the user pressed Home and the task went to the background. So the user sees only P when returning to the task.
If this attribute and allowTaskReparenting are both "true", any activities that can be re-parented are moved to the task they share an affinity with; the remaining activities are then dropped, as described above.