When referencing the value of a question in a formula, a default value can be set. The reference will return this value instead, if the referenced question is blank/null.
For instance, a question reference of the format:
[q1|No answer provided]
will return "No answer provided" when q1 is blank, and the content of q1 when it is not blank.
This can be used to cleanly default to a value, without needing an intermediate calculation in a Label or Variable question.