Can anyone suggest how to create single invoice for 2 orders using DOO

Summary:

What attribute we can use to create single invoice for 2 orders.

Requirement is for same customer having 2 orders needs to create single invoice - i understand that i need to write grouping rule and i am not finding which attribute or customer info i need to select in drop down while writing grouping rule.

Can someone please suggest what to select in drop down while writing grouping rule or how to achieve my requirement.

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