diff --git a/modules/libcom/src/error/errSymTbl.h b/modules/libcom/src/error/errSymTbl.h
index 66dcf9de8..89d74316c 100644
--- a/modules/libcom/src/error/errSymTbl.h
+++ b/modules/libcom/src/error/errSymTbl.h
@@ -35,13 +35,38 @@ typedef ERRSYMTAB *ERRSYMTAB_ID;
 extern "C" {
 #endif
 
+/** \brief Lookup message from error/status code.
+ * \param status Input code
+ * \param pBuf Output string buffer
+ * \param bufLength Length of output buffer in bytes.  Must be non-zero.
+ *
+ * Handles EPICS message codes, and "errno" codes.
+ *
+ * Copies in a mesage for any status code.  Unknown status codes
+ * are printed numerically.
+ */
 LIBCOM_API void errSymLookup(long status, char *pBuf, size_t bufLength);
+/** \brief Lookup message from error/status code.
+ * \param status Input code
+ * \returns A statically allocated message string.  Never NULL.
+ *
+ * Handles EPICS message codes, and "errno" codes.
+ *
+ * For any unknown status codes, a generic "Unknown code" message is returned.
+ *
+ * \since 3.16.1
+ */
 LIBCOM_API const char* errSymMsg(long status);
 LIBCOM_API void errSymTest(epicsUInt16 modnum, epicsUInt16 begErrNum,
     epicsUInt16 endErrNum);
 LIBCOM_API void errSymTestPrint(long errNum);
 LIBCOM_API int errSymBld(void);
-LIBCOM_API int errSymbolAdd(long errNum, const char *name);
+/** @brief Define new custom error code and associate message string.
+ *  @param errNum New error code.  Caller is reponsible for avoiding reuse of existing codes.
+ *  @param message New message.  Pointer stored.  Caller must not free pointed storage.
+ *  @return 0 on success
+ */
+LIBCOM_API int errSymbolAdd(long errNum, const char *message);
 LIBCOM_API void errSymDump(void);
 
 #ifdef __cplusplus