diff --git a/doc/advanced_usage.md b/doc/advanced_usage.md
index d768151..a3d5f03 100644
--- a/doc/advanced_usage.md
+++ b/doc/advanced_usage.md
@@ -322,6 +322,7 @@ Column  | Content
 
 #### variables `evtFault` and `evtFaultSpec`
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+In `eventList`, when the 4th column (event type) equals 6 (apply or clear faults, respectively), the index of the fault events in `evtFault` corresponds to the 5th column of `eventList`. In the current version, we only consider three-phase grounding faults. 
 ##### variable `evtFault`
 Column  | Content
 -------| -------------