Explanation will be appreciated

I was looking at a reference book and I think it laid out this general explanation in reference to the use of the in the dialogue below:

A speaker uses the when both people have the same thing or person in mind. The indicates that a noun is specific.

Dialogue:

A: Where is Jim?

B: He is in the livingroom. 

My question is "In the dialogue, do both people have the same thing or person in mind, which one?"

Sorry, if my question is not clear 

This is also called "common focus".

Both people have the same living room in mind, yes.

When B says "in the living room", he knows there is only one living room in the neighborhood of A.  That would have to be the living room being referred to.

Using the same reasoning, we say things like in the kitchen, in the refrigerator, on the porch, in the back yard, on the roof because there is only one of each of these associated with a typical home.

CJ

Give your ticket to the usher. (That would be the usher who is standing in front of us, with his hand out, waiting for the ticket.)

Comared to:
A: Honey, where's your ticket?
B: I don't know - I gave it to one of the ushers.

OR

The boss wants to see you. (We have the same boss so you know who I mean.)

compared to:

I enjoy working for a boss who doesn't micromanage (no specific person is meant)

OR My boss is giving me a hard time (You are being specific about what person)

OR I had a boss when I was in my early 20's who taught me so much about managing people (You have had more than one boss- this tell the person which one.)