There is mention of trading data which suggests to me that it might be to do with traders employed by Citigroup ie staff members whose job it is to use a block

There is mention of trading data which suggests to me that it might be to do with traders employed by Citigroup ie staff members whose job it is to use a block of the bank's money or outside investor's money to buy and sell shares, commodities and pretty much anything where they believe they can buy and sell to turn a profit.