Only the Acol bidding system is allowed (although substitutes may play their own system.)
We recognise that the Acol system is used in many countries, each with its own rules and expectations about alerts and announcements. It is wise, therefore, to assume that you should always self-alert your bids when playing in an international medium like BBO - even if your opponents appear to be from the same jurisdication as yourself. Your basic system should be announced at the beginning of every round and, where a bid may be misinterpreted by opponents, that bid must be alerted at the time of the bid (without waiting for a request).
All alerts, announcements and explanations should be made by the BBO alert mechanism or by private chat to the opponents - and never to the table. It is an infringement of the rules to alert or explain your bid in such a way as to give Unauthorised Information to your partner.
Acol is a Natural Approach Forcing system, and the choice of NT range or of major suit length does not change that concept, neither do changes to an evolving system necessarily mean that the result is not still Acol. The mere fact that a pair combines a strong NT with a 5+ major, will not determine that a pair is not bidding the Acol system.
PSYCHES and HUMS are not allowed. Please report players who bid psyches and HUMS to the TD.
Typically, you will announce your basic system at the beginning of each round. The following will be acceptable :
"Weak NT, 4 card majors, strong 2s." or
"Strong NT, 5 card majors, weak 2s."
with the normal alerting rules during the bidding.