By default, the "Simultaneous tasks" setting is disabled. Therefore, when the user taps a task on the "Today" tab if another task is running, that task will be stopped automatically.
With the "Simultaneous tasks" option enabled, the User can keep multiple tasks running simultaneously. For example, when a user starts a task on the "Today" tab, it will not stop the currently running task.
This option is helpful if users frequently track multiple tasks simultaneously. E.g., users read a book while commuting to work and want to track time spent on both.