在定语从句中，"that"作为关系代词的使用情况如下：
不定代词：
当先行词为anything, everything, nothing, all, few, much, several等不定代词时，只能使用"that"。
例句：
Is there anything that I can do for you？
She has everything that she needs for the trip.
形容词最高级：
当先行词是形容词最高级或被形容词最高级修饰时，只能使用"that"。
例句：
This is the most wonderful time that I have ever had.
He is the best student that our class has ever had.
序数词：
当先行词是序数词修饰时，只能使用"that"。
例句：
She is the first person that ever visited our country.
He is the second one to arrive.
特定限定词：
当先行词被the only, the very, the just等限定词修饰时，只能使用"that"。
例句：
He is the very man that I saw yesterday.
This is the only book that I have read this month.
既指人又指物：
当先行词既指人又指物时，可以使用"that"。
例句：
The man and his dog that were lost in the forest have been found.
主句以who或which开头：
当主句是以who或which开头的特殊疑问句时，定语从句用"that"引导，避免重复。
例句：
Who is the boy that is wearing a red shirt？
Which book is the one that you want to borrow？
总结：
在定语从句中，"that"的使用受到先行词类型和句子结构的限制。不定代词、形容词最高级、序数词、特定限定词以及既指人又指物的先行词通常只能与"that"搭配使用。此外，当主句以who或which开头时，也需使用"that"来引导定语从句。