Dollar UK上关于Mileage的说明的疑问

Mileage restrictions:

Unlimited mileage rates have no restrictions on the number of miles you are allowed to drive if you have proof of a round trip ticket. However, certain geographical restrictions may still apply. Miles per day rates specify a maximum number of miles that you are allowed to accumulate. A per mile fee may be charged for any additional miles. There may be minimum/maximum rental periods.


请大虾见红色字体，什么情况下有里程限制？具体什么是certain geographical restirction？


谢谢大虾