diff --git a/nix/module-eval/lib.nix b/nix/module-eval/lib.nix
index e85641d4..ad0a8a32 100644
--- a/nix/module-eval/lib.nix
+++ b/nix/module-eval/lib.nix
@@ -260,6 +260,8 @@ let
   # container's `config`; `true` once every unit's every `serviceConfig` value
   # is at WHNF. `seq` per value, not `deepSeq`: walking into a derivation-shaped
   # value's `override` self-reference overflows the stack.
+  # Covers `systemd.services` only (not sockets, timers or mounts), and only
+  # for checks that call it.
   forceServiceConfigs =
     containerCfg:
     let