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You will find this new feature upon the creation of the new email template for transactional email under Document types. Once you create a new model for supported document type you will be able to create a new "document model" by selecting the "Designer editor". See below.
What document types are supported?
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Merge tags are personalized variable pieces of information that is being applied to the email template dynamically respecting the data associated with (for example) an event name or individual receiving this email (e.g.: "Dear Mr Martin" - where "Mr" is "salutation" merge tag and "Martin" is "Contact first name" merge tag. Find more examples in the expansion section below per document type.
Supported document types with merge tags available
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