diff --git a/Stackage/Snapshot/Diff.hs b/Stackage/Snapshot/Diff.hs
index 582e70c..f041df4 100644
--- a/Stackage/Snapshot/Diff.hs
+++ b/Stackage/Snapshot/Diff.hs
@@ -18,6 +18,11 @@ type Version = Text
 
 type SnapshotDiff = HashMap PackageName VersionChange
 
+-- | Versions of a package as it occurs in the listings provided to `snapshotDiff`.
+--
+--   Would be represented with `These v1 v2` if the package is present in both listings,
+--   otherwise it would be `This v1` if the package is present only in the first listing,
+--   or `That v2` if only in the second.
 newtype VersionChange = VersionChange { unVersionChange :: These Version Version }
 
 changed :: VersionChange -> Bool