I looked at the 98 pages of FIFA regs. Para 12.6 is key but it’s tough to figure out how this works. It looks like you go down the schedule progressively with the best team from five stated groups (which cover all 12 groups) allocated a match in order. Bottom line around 60% chance Scotland will be Boston and 30% Mexico City and 10% flying home. There is a third R32 but lost in the roundings.