Is it correct, Although a we can use only one sum step per function in User Defined Function ?
Just tryed it and it seems you are right.
I also get this message:
Next question is why you would need more sum steps in your function?
One more question I know system can use Try/catch shape to send email but does it is necessary to have mail connector for that
That depends on which shape you insert after the catch shape.
If you set an exception shape then users who have subscribed to document/process errors will receive an alert mail.
If you don't want to use the exception shape and just use a normal email to notify users, then you will need to use a mail connector and a set properties shape to set the mail properties.
Retrieving data ...