Some dynamic content tags must always have a default value so that Email Autoresponder always has something to insert if the required information is not available. Tags that must have a default value are shown like this:
{TAGNAME default}
If the default value is optional, the tag is shown like this:
{TAGNAME <default>}
When “default” is shown in angled brackets <default>, it means that the default value is not required.
When writing your messages, try to think of what will appear in the message when the required information is not available. Don't assume your subscriber will enter the information you asked for!