From LimeSurvey Manual
Category separator (category_separator)
This attibute takes a char that will be considered by LimeSurvey as a category separator in answers.
If you set this to the char ":", and have an answer with the following text "Fruit: orange" and another with "Fruit: apple", then a category of "Fruit" is assumed and will contain 2 items "orange" and "apple". This will be displayed as categories/items in the dropdown list.