Key fields are automatically copied (based on name matching) from a parent list element into new child elements (e.g. created by using the scroll '+' button), in order to keep their prime keys consistent. If you want to inhibit this operation for certain fields, define the TabMenu function dontCopyKeyNames_<LIST NAME> to return an array of fields that should not be copied into new elements of the list named LIST_NAME
Example:
function dontCopyKeyNames_ENRL_FLD() {
return ['SEQ_NUM']
}