Usually if the item or object being inquired about belongs to the person answering the question, he will respond with "it is" or "it's" rather than "that is."
Does it mean there is no situation to reply that is
? No. Ouisch said 'usually'
For example, Fred is in his driveway washing a rusty, dilapidated old car. His friend John stops by and asks with a slight laugh "What the heck is that?" Fred replies with pride, "It's my new car." "New?" John scoffs. "It looks 10 years old at least." "Well, it's new to me, I saved my money for a whole year to buy this."
1. Does it mean it's wrong if Fred replied That is my new car? No.
2. Is what the heck an informal phrase? Yes.
d children pick up this expression? It's OK for children to say it. I've heard them say a lot worse.
4. Can I use it in formal writing? No.
5. Can I say it to my boss? Yes.