The <$DXCustom1$> etc are just placed in templates and the text/html within the associated field in the configuration document is placed there ("Custom Tags" tab in the config document).

There are also <$DXField1$> and <$DXField2$> - these are to allow you to have custom fields for each content document.

These fields are placed into template using the tags - the data is entered within the content document within the Miscellaneous tab (top two fields).

You can set the labels for these fields within the configuration document under the "Custom Tags" tab.