diff --git a/calibre-plugin/plugin-defaults.ini b/calibre-plugin/plugin-defaults.ini
index b774d2a9..428128a9 100644
--- a/calibre-plugin/plugin-defaults.ini
+++ b/calibre-plugin/plugin-defaults.ini
@@ -341,7 +341,9 @@ conditionals_use_lists:true
 
 ## Literal strings can be set into custom columns using double quotes.
 ## Each metadata=>column mapping must be on a separate line and each
-## needs to have one space at the start of each line.
+## needs to have one space at the start of each line.  There is also a
+## special case for Integer and Float type custom columns: The static
+## value "None" will clear the column.
 
 ## 'r_anthaver' and 'n_anthaver' can be used to indicate the same as
 ## 'r' and 'n' for normal downloads, but to average the metadata for