The entire universe of things that could be tested is typically too large to be covered within a given schedule, even more when multiple features are combined and the permutation grows exponentially.

可以进行测试的整个范围通常太大，难以在给定的时间内完成测试工作，当多个特性组合在一起时测试的范围将变得更大，其排列组合呈指数上升。