From LimeSurvey Manual
Code filter (code_filter)
Can be used to filter the label set codes of the current question based on the answer to a previous single choice question. The attribute was added for the purpose of being able to narrow down possible options - although it's of limited use since the answer of previous question is compared to the label code of the current one. So this will work with very short answers only since label codes have a limited size of five characters. You can either enter a SGQA identifier here or a fixed string.