It's the Latin initials for the Senate and the people of Rome. The Senate was mostly filled with Patricians, those who were aristocrats who claimed descent from the founders of Rome such as the Junii and the Julii, Marcus Junius Brutus came from the first and Caesar from the second.
To balance these you had the various councils and committees such as the Curia which elected army commanders and whose voters consisted of all citizens qualified to bear arms. There were also the Plebeian Tribunes who protected civil rights, to harm them or impede their duty was a capital offence, they also had the right of veto which they used to prevent unjust laws or arbitrary behaviour. Nice flag but is this supposed to be for a Roman alt hist it looks good with the purple