This means that there is a consensus on the requirements and understanding of the statement in given system. This is termed as common knowledge . Alex may not use the same language where English is not understood. The only reason for this is Alex knows that his statement would not be understood.
The similar concept applies in currency exchange. The only difference in currency exchange ecosystem is that the consensus is required in the complete ecosystem and not just the transacting parties. If I receive Fiat money or currency in exchange of goods and service, not only I but the complete ecosystem needs to recognize the same as money.