diff --git a/source/Network/Xmpp/IM/Roster/Types.hs b/source/Network/Xmpp/IM/Roster/Types.hs
index 6c63e82..7c03c98 100644
--- a/source/Network/Xmpp/IM/Roster/Types.hs
+++ b/source/Network/Xmpp/IM/Roster/Types.hs
@@ -7,7 +7,20 @@ import           Network.Xmpp.Types
 
 -- Note that `Remove' is not exported from IM.hs, as it will never be visible to
 -- the user anyway.
-data Subscription = None | To | From | Both | Remove deriving (Eq, Read, Show)
+data Subscription = None -- ^ the user does not have a subscription to the
+                         -- contact's presence information, and the contact does
+                         -- not have a subscription to the user's presence
+                         -- information
+                  | To  -- ^ the user has a subscription to the contact's
+                        -- presence information, but the contact does not have a
+                        -- subscription to the user's presence information
+                  | From -- ^ the contact has a subscription to the user's
+                         -- presence information, but the user does not have a
+                         -- subscription to the contact's presence information
+                  | Both -- ^ both the user and the contact have subscriptions
+                         -- to each other's presence information
+                  | Remove
+                  deriving (Eq, Read, Show)
 
 data Roster = Roster { ver :: Maybe Text
                      , items :: Map.Map Jid Item