This word is used with countable nouns to refer...

English as a Second Language English Vocabulary, Grammar and Idioms
1

Please read:

This word is used with countable nouns to refer to one of a number of things or people, when it does not matter which one.

“It” in this sentence refers to “one of a number of things or people”. It’s right?

Thanks
K

2

I consider it here as impersonal