Assistants referee at the u21 Fifa World Cup are allowed to use “referee spray” to mark position of free kick and defensive walls. It’s the first time during such an international tournament. “To have made a debut in a FIFA World Cup is undoubtedly the most important step we’ve taken together with the approval given us by IFAB (the International FA Board),” the spray’s Argentine inventor and producer Pablo Silva told Reuters. Silva hopes this referee gadget will be used during the 2014 World Cup in Brazil. The name of the product is Aerosol 9-15, referring to the 9.15 metres distance for the defensive wall. That’s why some call it…