--- a/doc/FR1-library-API	Fri Sep 20 07:27:18 2024 +0000
+++ b/doc/FR1-library-API	Sat Sep 21 20:48:58 2024 +0000
@@ -374,11 +374,20 @@
 and the TFO transform is that the former emits linear PCM output, whereas the
 latter emits 33-byte GSM-FR codec frames to be sent to call leg B downlink.
 
+The return value from gsmfr_tfo_xfrm_main() is the same as that of
+gsmfr_fulldec_rtp_in(): 0 if the the RTP input was considered good or -1 if it
+is invalid.  In the case of invalid RTP input that produces -1 return value,
+gsmfr_tfo_xfrm_main() calls gsmfr_preproc_bfi(), just like how
+gsmfr_fulldec_rtp_in() calls gsmfr_fulldec_bfi() under the same conditions.
+
 If DTXd is in use, then the call to gsmfr_tfo_xfrm_main() needs to be directly
 followed by a call to gsmfr_tfo_xfrm_dtxd(), operating on the same output buffer
 with the same state structure.  The output will then be changed to SID when
 appropriate for the current state.
 
+The return value from gsmfr_tfo_xfrm_dtxd() is the SP flag of GSM 06.31: 1 if
+the output frame is speech or 0 if it is SID.
+
 TFO transform homing
 --------------------