"dummy" it or not

0Hi,02br
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00Can you tell me if my assertions are correct in regard to the "its" acting in a "dummy" capacity?02br
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001. Someone is calling me:02br
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01i00 Who is it?02i02br
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01i00It is me.02i00 -- I think in this case, 'It' is not functioning in a dummy capacity because 'it' refer to a specific one as indicated by the question 'Who is IT'?02br
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002. Knocking the door:02br
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01i00(Knock, knock) who is it?02i02br
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01i00It's me02i00 -- Again, 'It' seems to have a legitimate place in this sentence and acting as a subject. 02br
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003. Identiying who a possible supect is:02br
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01i00OK, do you tell (point to) us the guy you saw who committed robbery from those five guys in line?02i02br
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01i00Yes, sir, it's (or that's) him.02i00 -- Here, 'it' is not functioning as a dummy, but rather as a subject.02br
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00As I write what I think, it's mostly guesses. Do you have some good tips? 0-