Relative Pronoun and Referring Noun

[vinaynp]

From the document GMAT Verbal Notes by Sumit:

c. Relative pronouns are often used incorrectly today.
1) Referring to things or animals – that, which
2) Referring to people—who, whom
3) They – be careful that you don’t use this unless you’re positive there is a referring noun. Today we often use “they” to replace the use of a proper noun which it is not. It’s a Pronoun.

Can somebody explain the meaning of referring nouns here with examples?

Also, Manhattan SC says that you can use the pronoun that for people or things.

However, the above notes says the modifier that for things or animals. Can somebody throw some more light on this?

[nikhilgirdhar]

that/which should be used in reference to non living objects...
we need to check on what Manhattan SC says...

[vinaynp]

that/which should be used in reference to non living objects...
we need to check on what Manhattan SC says...

The below content is from Modifiers Strategy chapter of Manhattan SC and no copyright infringement is intended.

The pronoun who introduces phrases that modify a person or a group of people, while the pronoun which introdcue phrases that modify things. The pronoun that can be used to modify either people or things.