The reason that they quarreled is not clear.他们吵架的原因还不清楚.
这个句子that引导的是同位语从句.同位语从句和定语从句的区别在于,这个that是否在句子中充当成分.
举两个例子：
1.The news that he will come back is true.
2.The news that he told me is true.
第一个是同位语,因为that不在句子中充当成分.
第二个是定语从句,因为that在句子主语中充当told的宾语,引导作为news的后置定语.