From LimeSurvey Manual
If the value of the relevance equation is "1" or "true", the question is "relevant" in the survey context, i.e. it is shown to the survey participant. If not, the question is hidden. Any survey question allows you to specify a relevance equation. This function is the successor of conditions and supports much more complex conditional logic.
Whenever you save the relevance equation, it is evaluated and syntax-highlighted. Any errors will be color coded so that you can quickly detect and fix them.
- Any equation that makes use of the Expression Manager syntax, without surrounding curly braces.
Here are good examples of syntax highlighting.