你先对比一下这个句子和并列句的区别：

She did not discover the coincidence of the market with the fair till she had reached the place, by which time it was close upon dusk. 非限制性定语从句。

She did not discover the coincidence of the market with the fair till she had reached the place, and by that time it was close upon dusk. 并列句。

并列句中的by that time= by the time he had reached the place. that time指她到达该地的时间即前面分句中谓语动词发生的时间。that为指示代词。如果把并列句改为主从复合句，则只需删去并列标记and, 用关系限定词which取代指示限定词，就得到上述非限制性定语从句。在该定语从句中，which为限定词，不要仅仅说which指代什么，而应考虑which time指代什么。

It is a statute （that）everybody concedes is flagrantly unconstitutional.

这是一个嵌入式定语从句。即定语从句本身是一个带有宾语从句的主从复合句，关系代词that在其中的宾语从句中作主语。即that is flagrantly unconstitutional 是 everybody concedes的宾语从句。这种从句中关系代词作嵌入分句的主语是可以省略的。