What is the main difference when creating a contact process and and assigning a task to process manager or pick user?

If assign to process manager, does the process manager needs to complete the task?

If pick another user, then that will create an acvity for that particular user to complete the task? if that is the case, can the process manager still see that user's activity to keep track if task was completed? If assigned to a user, the process manager is still the one who needs to scan the process?

Thank you,