You may have noticed that BBO no longer defaults to posting a
convention card if you and your partner don't have one.
If I see that a partnership does not have a posted convention card,
as a director I will assume they are playing the Standard American
Yellow Card (SAYC), and rule accordingly. In the Open games,
conventions not listed on the SAYC card will not be allowed.
In the limited games I'll ask pairs to play SAYC only until they
have a card posted.
Luckily it's not very difficult to create a card online. Here's a
great 4-minute video by Sylvana Morici on how to make a convention
card on BBO: