Text field, material , material description show blank in FBL5N

Dear expert,

Could anyone advise me , the text field in FBL5N get from which field in Sales Order ? Current set up I assigned the assignment field in FBL5N equal to Sales Order and Reference field equal to billing document. So whenever I post invoice from Sales Order those two information is getting automatically . I am wondering as well since other field in FBL5N like material or material description show blank while those information existing in Sales Order. Thank you.