diff --git a/operate.lisp b/operate.lisp
index b91ddb23c02e3e74b498b1ce3ba2602d1be85d85..a1dda00be1b00af724bde6a3913dbca80079dd21 100644
--- a/operate.lisp
+++ b/operate.lisp
@@ -156,7 +156,9 @@ for how to load or compile stuff")
   (defvar *modules-being-required* nil)
 
   (defclass require-system (system)
-    ((module :initarg :module :initform nil :accessor required-module)))
+    ((module :initarg :module :initform nil :accessor required-module))
+    (:documentation "A SYSTEM subclass whose processing is handled by
+the implementation's REQUIRE rather than by internal ASDF mechanisms."))
 
   (defmethod perform ((o compile-op) (c require-system))
     nil)