Where is this Static Text used - "Description of the order for the customer (in email) (send manually) "

Where is this Static Text used - "Description of the order for the customer (in email) (send manually) "

There is a static text "Description of the order for the customer (in email) (send manually) "

Where is this used? I have not found any option to send email manually? Is that an addon?

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Aw: Where is this Static Text used - "Description of the order for the customer (in email) (send manually) "

1. Configuration / checkout
Send invoice manually: Y

2. after create order
order / order xxxx / send mail