New member parser documentation.

MEMBERS.md

@@ -60,8 +60,32 @@ line numbers as the value for such key.
 
 The result is a valid dictionary of keys and multikeys.
 
-Member File Grammar
--------------------
+Member File Interpreter - Passes 3+
+-----------------------------------
+
+After getting a valid dictionary from the 2nd pass, the data gets interpreted.
+
+### Pass Markers
+
+All start/stop (student and suspend at the moment) markers are joined
+into lists of markers, retaining source position information.
+
+### Info
+
+The student and suspend lists of markers are converted into a lists of
+periods.
+
+The joined key is converted into a month value.
+
+Card and desfire lists are parsed to get lists of card id and optional
+comment.
+
+Credit list is parsed to get a list of amounts and optional comments.
+
+Member Record Finalization
+--------------------------
+
+If there is no ```'joined``` key, default month is substituded.
 
 Member File Schema
 ------------------