Examine the following usages from English and try to understand why sometimes it can refer to an individual and why sometimes it cannot and a personal pronoun, he or she, is necessary:

(1)     (a)     A1:     Who's that knocking on the front door?
                  A2:      There's someone taking your car!
         (b)     B1:      It's Elmo.
         (c)     B2:   %He's Elmo.

(2)     (a)     A1:     Who's the man in the kitchen?
                  A2:      There's a man in the kitchen!
         (b)     B1:      It's the plumber.
         (c)     B2:      He's the plumber.

(3)     (a)     A1:     Who's Robert?
                  A2:      I don't know who this person Ronny is you're talking about.
         (b)     B1:  %It's my best friend.
         (c)     B2:      He's my best friend.

The symbol % means that the utterance following it has a meaning which is probably not suitable for that context. In (1) - (3), both question A1 and statement A2 have the same effect. In (1), A1 and A2 are both compatible with the B1 response, but not the B2. In (2), A1 and A2 are both comfortable with either response B1 or B2. In (3). both A1 and A2 are suitably matched by the B2 response, but not the B1.

Compare the A1 question of (2) with the A1 question of (4) and its possible answers:

(4)     (a)     A:       Who's the murderer?
         (b)     B1:      It's the plumber.
         (c)     B2:   %He's the plumber.

The pairing of (2) with (4) suggests that the difference between it and he may lie in part in the questions Who's the man in the kitchen? and Who's the murderer? If the responses of (1) - (4) stand as marked (by % or not), then part of the answer to what it means to choose it or she/he must lie in the difference in the respective who-questions of (1) and (4) - and in the A2 statements of (1) - (3).

What is the difference between asking Who's that knocking on the front door? and Who's the man in the kitchen?