Why is the national debt a "gilt bond" but not a national debt?

As early as the17th century, with the approval of Parliament, the British government began to issue government bonds with tax guarantee to pay principal and interest, which has high credibility. The British government bonds issued at that time had Phnom Penh, so they were called "Phnom Penh bonds". In the United States, bonds rated as the highest credit rating (AAA) by authoritative credit rating agencies are also called "Phnom Penh bonds". Later, the term "Phnom Penh bond" generally refers to all bonds issued by the central government, that is, national debt. The central government is a symbol of state power, which takes the state's tax capacity as a guarantee to repay the principal and interest of the national debt. Investors generally don't have to worry about the repayment ability of "Phnom Penh bonds". In order to encourage investors to buy government bonds, most countries stipulate that investors in government bonds can enjoy tax incentives or even tax exemption in debt interest. Therefore, "Phnom Penh Bond" is very popular among investors, with strong liquidity, and is widely used as mortgage and guarantee. However, due to the low risk, good security and liquidity of national debt, its interest rate is generally lower than other types of bonds. There are generally two situations in which the government issues bonds; One is to raise funds for national economic construction and issue bonds, which are called "construction bonds", such as "people's victory bonds" issued in the early stage of China's construction; The second is to issue bonds to make up the budget balance, which is called "deficit bonds". For example, the national debt issued by China since 198 1 belongs to this nature. In some cases, it is a positive development strategy to increase investment in economic construction by borrowing public debt; Using public debt to make up the budget balance is sometimes an effective means to balance fiscal revenue and expenditure. However, the scale of national debt must be controlled within a moderate range, otherwise there will be a debt crisis.