Treasury buys 11% of debt. The US Government owns 27%. When the government borrows money it uses a vehicle like a bond. When it pays interest, it all goes to Treasury. So, actually the government is borrowing money from all the bond holders and creating debt by expenditures paid from Treasury and when it pays interest that also goes to Treasury. But, only 11% of debt is held by Treasury so in the scope of things there is not much borrowing going on with Treasury. But, they do see interest as a book transfer