call() deserves a good top-comment.

src/server.c

@@ -2164,7 +2164,43 @@ void preventCommandReplication(client *c) {
     c->flags |= CLIENT_PREVENT_REPL_PROP;
 }
 
-/* Call() is the core of Redis execution of a command */
+/* Call() is the core of Redis execution of a command.
+ *
+ * The following flags can be passed:
+ * CMD_CALL_NONE        No flags.
+ * CMD_CALL_SLOWLOG     Check command speed and log in the slow log if needed.
+ * CMD_CALL_STATS       Populate command stats.
+ * CMD_CALL_PROPAGATE_AOF   Append command to AOF if it modified the dataset
+ *                          or if the client flags are forcing propagation.
+ * CMD_CALL_PROPAGATE_REPL  Send command to salves if it modified the dataset
+ *                          or if the client flags are forcing propagation.
+ * CMD_CALL_PROPAGATE   Alias for PROPAGATE_AOF|PROPAGATE_REPL.
+ * CMD_CALL_FULL        Alias for SLOWLOG|STATS|PROPAGATE.
+ *
+ * The exact propagation behavior depends on the client flags.
+ * Specifically:
+ *
+ * 1. If the client flags CLIENT_FORCE_AOF or CLIENT_FORCE_REPL are set
+ *    and assuming the corresponding CMD_CALL_PROPAGATE_AOF/REPL is set
+ *    in the call flags, then the command is propagated even if the
+ *    dataset was not affected by the command.
+ * 2. If the client flags CLIENT_PREVENT_REPL_PROP or CLIENT_PREVENT_AOF_PROP
+ *    are set, the propagation into AOF or to slaves is not performed even
+ *    if the command modified the dataset.
+ *
+ * Note that regardless of the client flags, if CMD_CALL_PROPAGATE_AOF
+ * or CMD_CALL_PROPAGATE_REPL are not set, then respectively AOF or
+ * slaves propagation will never occur.
+ *
+ * Client flags are modified by the implementation of a given command
+ * using the following API:
+ *
+ * forceCommandPropagation(client *c, int flags);
+ * preventCommandPropagation(client *c);
+ * preventCommandAOF(client *c);
+ * preventCommandReplication(client *c);
+ *
+ */
 void call(client *c, int flags) {
     long long dirty, start, duration;
     int client_old_flags = c->flags;
@@ -2229,7 +2265,7 @@ void call(client *c, int flags) {
          * set for replication / AOF propagation. */
         if (dirty) propagate_flags |= (PROPAGATE_AOF|PROPAGATE_REPL);
 
-        /* If the command forced AOF / replication of the command, set
+        /* If the client forced AOF / replication of the command, set
          * the flags regardless of the command effects on the data set. */
         if (c->flags & CLIENT_FORCE_REPL) propagate_flags |= PROPAGATE_REPL;
         if (c->flags & CLIENT_FORCE_AOF) propagate_flags |= PROPAGATE_AOF;