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Not exclusively. Это было нечто! Некто в кожаном пальто...

OK, I'll try to put the explanation in series with Боб Уайтман's ones.

некто/нечто conveys an idea of something speaker has...

There is one more way in Russian to say "something": "нечто".

There are also некто, некогда with corresponding meanings.

I am not sure how to comment the difference though.

I think it can always be avoided.



:good:

+1

In most cases there is no difference at all. If it is, the difference is very delicate and native speakers probably will not agree about it. :)

что-нибудь can mean "whatever" while что-либо can...

I like the following interpretation:
"Что-нибудь" - something unknown (the choice in not made yet)
"кое-что" - something certain ("you know what..." )