-The first is for the "concept index" and the second is for the "options index".
-Not all forms of output distinguish between these - sometimes there is just one
-index.
+The first is for the "concept index", the second is for the "options index",
+and the third is for the "variables" index. The secondary text is of course
+optional. Not all forms of output distinguish between these - sometimes there
+is just one index. For the concept index, it is also possible to set "start"
+and "end" markers so that the entry lists a range of pages. This is how to do
+that:
+
+ .scindex IID "primary text" "secondary text"
+ <intervening text, should be several pages>
+ .ecindex IID
+
+The IID must be some unique string that ties the entries together.