There are no "v0..v7" variables. Between the parentheses in a function definition is the list of arguments that the function takes. "v0..v7" just means that the function takes 8 arguments - their names are up to you.

TradeVar is used for storing a trade-dependent value between TMF calls. It's like a static variable, only difference is that static variables would be the same for all trades, while a TradeVar can have an individual value per trade.