The use of these two commands is discussed extensively in Define layout in a base template and Implement an optional option with ReplaceField.
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Sometimes a letterhead needs to dictate not just things like the company’s logo and fairly static items like the directors’ names, but also the position of the addressee information and data elements like ‘Your Reference’ and ‘Our Reference’, for example.
Sometimes it would be useful to be able to customize a source template for more than just the letter body. For example, it may be that the Data Set contains data for more than one set of addressees and the source document needs to be able to indicate which of these addressees is to be included on the base template.