If you want to use default Merge Tag in your email campaign because some of your recipients do not have these data (First Name, Last Name, Full Name, etc) in their records, then you can set a default value instead. The Default value can be inserted like this in your email campaign:
{{ first_name or “default value” }}
Here is a real example:
{{ first_name or “Hello!” }}
Here we have mentioned the value inside the double quote.