The possible values of face are FDFNormalAP, FDFRolloverAP and FDFDownAP.
The fdf_get_ap() function gets the appearance of a field (i.e. the value of the /AP key) and stores it in a file. The possible values of face are FDFNormalAP, FDFRolloverAP and FDFDownAP. The appearance is stored in filename.