The first parameter is the list of tags for the widget to display when it is first loaded. If you are displaying return delimited data from a single field, enter the field. If it is data from related records, you could use the list function. If it is more complicated data, you could run a script to gather the data into a global field or variable.
One thing to be aware of, is if you use record data as the initial list, any change to the data in the records will cause the calculation in the web viewer url definition to re-evaluate and update the widget. For the most part this is fine, and actually a convenient and quick way to make sure if the underlying data is changed somewhere else your widget is displaying the most up to date information.