Which bidding system is assumed by default at Bridge Base Online (for humans)?
I asked in chat several times in different games (ten times I think) and nobody answered.
Simple answer- No system- its an open community. If high level players like JEC don't need to specify a system why would anyone else. Partial answer- Most English speaking bridge players are American so can assume they play some sort of Standard American- they assume you are psychic so you know what they play.
Playing with random partners on BBO is pretty frustrating. You will find a lot of "advanced" players playing "SAYC" who:
These bad habits aside, there's still the problem of unfamiliarity with each other's conventions, etc. My advice is: use minimal gadgetry, pass a lot, when you know that someone at the table is lying, don't automatically assume it isn't your partner, and, eventually, when you find a partner who, if not necessarily good, doesn't violate (1)-(8) above, add them as "friend" and then you can play with them the next time you are both on BBO instead of going fishing in the void.