Supply with related parties?

Both are considered supply. There is a difference in valuation method. You will have to elaborate your question

The reason is simply that in case of related persons both persons can control the value of  transactions for mutual gains. In case of unrelated persons logically no one will sell at zero price.

A father can transfer goods to his son without value.

But the same person will not ordinarily sell without consideration

So the relation of a person is influencing the price therefore a loss to the government.