Adding merge fields is a way to personalize a document with information from the data source.
You can insert merge fields from the email builder using the Insert Custom Fields option as shown below:
In the example above, %%FirstName%% and %%birthday__c%% tokens will automatically be replaced with Prospect's first name and birth date in the actual email.
Default values may be used when no value exists at the time of merge. To use a default value follow this form:
%%FieldName:default=Default Value Here%%
Default values work for standard or custom fields. For example, suppose you're sending out an email campaign and you want to personalize the email with the person's first name by saying something like "Hey Mike". However, if the person did not have a first name in the system you could say something like "Hey There". Here's how you'd do it:
You may learn more about Data Merge in our documentation here.