EBU Mag August 2018 - Sleuth's Quiz , page 9 - Re: Permitted Understandings
I think the article "Sleuth's Quiz" bottom left paragraph "Here's a useful tip", advises readers to use a prohibited dual meaning signal. It says when following suit with 5+ cards you can signal odd to encourage or play an even card to give suit preference.
EBU Blue Book - 7 F 3 Dual meaning signals
Dual meaning signals (when following suit) are not permitted.
Examples of prohibited dual meaning signals:
(a) One message (typically attitude) is given according to whether the card played is odd or even; a different message (typically suit preference) is given according to whether the card played is high or low.
Gordon have the rules been amended?