diff --git a/pressure-vessel/runtime.c b/pressure-vessel/runtime.c
index eccbe7760827130e3cd651fa0ceeb46ca3919a8c..89e950771ab884d621e2092a40f922413004d229 100644
--- a/pressure-vessel/runtime.c
+++ b/pressure-vessel/runtime.c
@@ -145,6 +145,9 @@ path_visible_in_provider_namespace (PvRuntimeFlags flags,
   while (path[0] == '/')
     path++;
 
+  /* In a Flatpak subsandbox, the provider is /run/parent, and
+   * /run/parent/app in the subsandbox has the same content as /app
+   * in Steam. */
   if ((flags & PV_RUNTIME_FLAGS_FLATPAK_SUBSANDBOX)
       && g_str_has_prefix (path, "app")
       && (path[3] == '\0' || path[3] == '/'))