--- a/doc/Binary-file-format	Sun Apr 16 08:25:19 2023 +0000
+++ b/doc/Binary-file-format	Sun Apr 16 19:00:46 2023 +0000
@@ -9,7 +9,7 @@
 
 - a 33-byte GSM FR frame in libgsm/RTP format, 0xD signature
 - a 31-byte GSM EFR frame in RTP format (ETSI TS 101 318), 0xC signature
-- a 2-byte Themyscira-extension BFI marker, 0xBF signature
+- a 2-byte Themyscira-extension BFI marker, 0xBF signature, see below
 
 File reading functions begin by reading only one byte; this byte, once decoded,
 tells us how many more bytes need to be read, and frame synchronization is thus
@@ -30,3 +30,16 @@
 our low-level binary file reading function and gsmrec-dump will grok such mixing
 just fine, but each higher-level test program (beyond gsmrec-dump) is expected
 to be written for only one codec, either FR or EFR.
+
+BFI marker format
+=================
+
+Every 20 ms frame in our gsmx files is either a good FR/EFR frame or a BFI (Bad
+Frame Indication) marker.  The BFI marker format used in our gsmx file format is
+the same format which we (Themyscira Wireless) previously used in our GSM RAN
+RTP transport, before switching to our current TRAUlike RTP format.  This BFI
+marker format is quite simple:
+
+byte 0: 0xBF signature;
+byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
+        section 6.1.1 in both documents; other bits are reserved.