What is unclear from the docs ?
The “singleTask” and “singleInstance” modes also differ from each
other in only one respect: A “singleTask” activity allows other
activities to be part of its task. It’s always at the root of its
task, but other activities (necessarily “standard” and “singleTop”
activities) can be launched into that task. A “singleInstance”
activity, on the other hand, permits no other activities to be part of
its task. It’s the only activity in the task. If it starts another
activity, that activity is assigned to a different task — as if
FLAG_ACTIVITY_NEW_TASK was in the intent.