which在句子中的有什么意义？这是什么从句？

When students go to college, they have to live in the dormitory which often holds four students.

which在句子中引导定语从句，定语从句中心词是dormitary,which often holds four students都是说明解释dormitory的。
本句意思：当同学们读大学时，他们不得不住在可以容纳4个人的学生宿舍里。
英语定语从句中which的用法有几种?
定语从句的关系代词 Which的用法:若先行词指物则代替先行词在定语从句中充当主语或宾语.
This is the pen which was given by my friend.
先行词 指物代替先行词在定语从句中充当主语
This is the pen which my friend gave to me.
先行词 指物代替先行词在定语从句中充当宾语
下面是他和that在定语从句中的区别及用法：
that \which在代物时常常可以通用,但有时只宜于用which ,不用that
(1) 关系代词前有介词时.
This is the hotel in which you will stay.
(2) 如有两个定语从句,其中一句的关系代词是that,另一句宜于用which.
Let me show you the novel that I borrowed from the liberary which was newly open (新开放) to us.
注意2that \which在代物时常常可以通用,但有时只宜于用that ,不用which.
(1) 先行词是形容词最高级或者它的前面有形容词最高级时
This is the best that has been used against air pollution(反对空气污染) in cities.
English is the most difficult subject that you will learn during these years.
(2) 先行词是序数词,或它的前面有一个序数词时
He is the last person that I want to see.
(3) 主句中已有疑问词时
Which is the bike that you lost?
which和that的区别？

which与that两者都可指物，常可互换。其区别主要在于：
1. 引导非限制性定语从句时，通常要用which。如：She received an invitation from her boss, which came as a surprise. 她收到了老板的邀请，这是她意想不到的。
2. 直接放在介词后作宾语时，通常要用which。如：The tool with which he is working is called a hammer. 他干活用的那个工具叫做锤子。
3. much, little, none, all, few, every(thing), any(thing), no(thing) 等时，通常用that。如：There was little that the enemy could do but surrender. 敌人无法,只有投降了。All [Everything] that can be done must be done. 凡能that can be done must be done.
4. 当先行词有the very, the only, the same 等修饰时，通常用that。如：This is the only example that I know. 我知道的例子只有这一个。Those are the very words that he used. 那是他的原话。
5. 当先行词有形容词最高级或序数词(包括last, next等)等修饰时，通常用that。如：。如：This is the best dictionary that I’ve ever used. 这是我用过的最好的词典。The first thing that you should do is to work out a plan. 你应该做的第一件事是订个计划。