diff --git a/src/include/lwip/priv/tcp_priv.h b/src/include/lwip/priv/tcp_priv.h
index 9987657a..baa54122 100644
--- a/src/include/lwip/priv/tcp_priv.h
+++ b/src/include/lwip/priv/tcp_priv.h
@@ -261,12 +261,12 @@ struct tcp_seg {
   u8_t  chksum_swapped;
 #endif /* TCP_CHECKSUM_ON_COPY */
   u8_t  flags;
-#define TF_SEG_OPTS_MSS         (u8_t)0x01U /* Include MSS option. */
+#define TF_SEG_OPTS_MSS         (u8_t)0x01U /* Include MSS option (only used in SYN segments) */
 #define TF_SEG_OPTS_TS          (u8_t)0x02U /* Include timestamp option. */
 #define TF_SEG_DATA_CHECKSUMMED (u8_t)0x04U /* ALL data (not the header) is
                                                checksummed into 'chksum' */
-#define TF_SEG_OPTS_WND_SCALE   (u8_t)0x08U /* Include WND SCALE option */
-#define TF_SEG_OPTS_SACK_PERM   (u8_t)0x10U /* Include SACK Permitted option */
+#define TF_SEG_OPTS_WND_SCALE   (u8_t)0x08U /* Include WND SCALE option (only used in SYN segments) */
+#define TF_SEG_OPTS_SACK_PERM   (u8_t)0x10U /* Include SACK Permitted option (only used in SYN segments) */
   struct tcp_hdr *tcphdr;  /* the TCP header */
 };