By default, all the activities in an application have an affinity for each other — that is, there's a preference for them all to belong to the same task. However, an individual affinity can be set for each activity with the taskAffinity attribute of the
The FLAG_ACTIVITY_NEW_TASK flag
As described earlier, a new activity is, by default, launched into the task of the activity that called startActivity(). It's pushed onto the same stack as the caller. However, if the Intent object passed to startActivity() contains the FLAG_ACTIVITY_NEW_TASK flag, the system looks for a different task to house the new activity. Often, as the name of the flag implies, it's a new task. However, it doesn't have to be. If there's already an existing task with the same affinity as the new activity, the activity is launched into that task. If not, it begins a new task.
The allowTaskReparenting attribute
If an activity has its allowTaskReparenting attribute set to "true", it can move from the task it starts in to the task it has an affinity for when that task comes to the fore. For example, suppose that an activity that reports weather conditions in selected cities is defined as part of a travel application. It has the same affinity as other activities in the same application (the default affinity) and it allows reparenting. One of your activities starts the weather reporter, so it initially belongs to the same task as your activity. However, when the travel application next comes forward, the weather reporter will be reassigned to and displayed with that task.
If an .apk file contains more than one "application" from the user's point of view, you will probably want to assign different affinities to the activities associated with each of them.