For example you wanted to set up email notifications to be sent when purchase orders are approved.
or you did not find the task available in task subscriptions.
SAP Business ByDesign
You have to set up work flow rules for those scenarios that does not exist under Home ->Subscribe to tasks
Go to Application user management work center
Go to work flow rules view
Follow steps given in SCN community blog in below link
If you have not found what you are looking for then contact your partners to help develop it for you.
Refee the help center document
also refer KBA : 2658668 - How to set up an E-Mail notification for Tasks
Work flow rule, task subscription, email notification, task notification,new notification , KBA , AP-POP-PO , Purchase Order , AP-SIP , Supplier Invoice Processing , Problem