SO = Stock Option not sure of the other
I have been trading since the 1960s.
Never came across "SOUS Trader" before.
US probably means USA Options.
I think wardaddy meant “SOES” and he is talking about this:
“SOES Bandits: Who They Were, What They did”
“SOES Bandits was the nickname given collectively to a group of individual investors who—following the 1987 Market Crash—exploited the Nasdaq’s Small Order Execution System (SOES) for day trading. The strategies and tactics they devised back then created the earliest form of what’s now called high-frequency trading. While the bandit’s average profit per trade is small, they make up for it by trading dozens or hundreds of times per week. Bandits usually establish a position prior to most market makers having updated their quotes and lay off the positions at favorable prices.
“An interesting side note to the SOES bandit story was their ability to profit off professional market markers despite a comparative disadvantage in information capabilities. Because bandits reap the profits and bear the losses from their trades, perhaps they were sharper given greater incentives to perform better than traditional market-making firms.
more here:
https://www.investopedia.com/terms/s/soesbandits.asp