diff --git a/t/lib/Setup.pm b/t/lib/Setup.pm
index 28181e96..151e3d61 100644
--- a/t/lib/Setup.pm
+++ b/t/lib/Setup.pm
@@ -20,6 +20,9 @@ our @EXPORT = qw(test_init hydra_setup nrBuildsForJobset queuedBuildsForJobset
 #
 #  * hydra_config: configuration for the Hydra processes for your test.
 #  * nix_config: text to include in the test's nix.conf
+#  * use_external_destination_store: Boolean indicating whether hydra should
+#       use a destination store different from the evaluation store.
+#       True by default.
 #
 # This clears several environment variables and sets them to ephemeral
 # values: a temporary database, temporary Nix store, temporary Hydra