When a Task is executed the steps are carried out as if the creator of the Task was doing the steps manually.
E.g. If a Task sending messages to Reviewers is created by Peter Jones, then the messages are only sent if Peter has the permissions to send messages, otherwise it will fail.
If the Task has any child tasks then the child tasks will be executed using the same permissions as the Parent Task, even if the Child Task has a different creator.
If that Child Task has its own Child Tasks then these too are executed using the same permissions as the original Parent Task.
So, the permissions are taken from the very first Task that is executed in a series of combined tasks. All children and grandchildren etc run with these original permissions.