This is the use case where activity A replaces activity B in a different application. This situation normally happens because activity A is better at doing the job than activity B. In other words, A and B are equivalent enough that A can replace B. This case stands in contrast with re-using an activity, where A and B are quite different activities and supplement each other.
In this example, the user has downloaded a replacement for the Phone Ringtone activity, called Rings Extended. Now when they go to Settings, Sound & Display, Phone Ringtone, the system presents them with a choice between the Android System's ringtone activity and the new one. This dialog box has an option to remember their choice "Use by default for this action".
When they choose "Rings Extended", that activity loads, replacing the original Android ringtone activity.
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