I can see that basketno value is used in many places like
case "addtobasket":
ShopAddToBasket(GetShopProductArg,WebGetArg("qty"),WebGetArg("config"),WebGetArg("basketno"));

but I cannot find where it comes from. WebGetArg() gets the value from the POST or GET as far as I understand, but I cannot find "basketno" as an anywhere in the code.