From LimeSurvey Manual
Numbers only (numbers_only)
If you enable this option, the participant can only enter numbers in the text box(es).
For the equation question types, this setting indicates that the result could only be a number, not a string. This will guarantee proper calculations/conversions in follow-up equations regarding the decimal mark.
Behavior by question type
- Default: If the subject enters a value that is not a number, that value is immediately cleared from the text box so that the subject can enter an appropriate value.
- Array (Texts): If the numbers only option is disabled, the "Show totals for" and "Show grand total" options will be overruled, while the total text boxes will not be displayed.
- Equation: Enabling this option will force the equation results to be converted to a numeric value. If the equation result is not a number (and not blank), the equation will return NaN, being saved as an empty string in the response table.
- Off (default)