What the holder owns is a share of a stream, not a loan. A pass-through certificate entitles its owner to a fractional interest in every dollar the pool collects. That is a different instrument from a bond. A corporate bond pays interest on a fixed schedule and returns the whole principal on one known date; a mortgage pass-through returns principal continuously, in amounts nobody can predict, because each month's principal is whatever the borrowers happened to repay. Scheduled amortization is the predictable part. Everything else is behavior.
Prepayment risk is the concept the whole market is organized around. A homeowner may repay early at any time, by refinancing, selling, or simply paying extra. When interest rates fall, refinancing surges and the pool pays down fast, handing the investor principal precisely when the only available reinvestment is at the new, lower rate. When rates rise, refinancing stops and borrowers keep loans they would not be able to replace, so the pool's life stretches out and the investor is locked into a below-market rate for longer. The two together are why a mortgage-backed security tends to gain less in a rally than a comparable bond and lose more in a selloff. That asymmetry has a name in the trade, negative convexity, and it is the reason these securities ordinarily yield more than Treasury debt of similar credit quality: the extra yield is payment for handing the timing decision to the borrower.
A guarantee removes credit risk and leaves the timing risk untouched. This is the single most common misreading. An agency guarantee promises that the holder receives principal and interest even if borrowers default. It promises nothing about when. In fact a default on a guaranteed pool usually accelerates the cash flow, because the guarantor buys the loan out of the pool and the investor receives that principal immediately, which is indistinguishable from a prepayment.
The three levels of guarantee, and the one that surprises people. Ginnie Mae does not issue securities; it guarantees securities that approved issuers create from pools of loans insured or guaranteed by federal housing programs, and 12 U.S.C. 1721(g)(1) provides that "the full faith and credit of the United States is pledged to the payment of all amounts which may be required to be paid under any guaranty under this subsection". Ginnie Mae is not a government-sponsored enterprise: 12 U.S.C. 1717(a)(2)(A) made it "a body corporate without capital stock" inside the Department of Housing and Urban Development. Fannie Mae and Freddie Mac, which are government-sponsored enterprises, guarantee their own securities with their own balance sheets, and 12 U.S.C. 1719(d) requires Fannie Mae to state in the securities themselves that they "are not guaranteed by the United States and do not constitute a debt or obligation of the United States or any agency or instrumentality thereof other than the corporation". Private-label securities, issued by banks and finance companies from loans no agency will buy, carry no third-party guarantee; they manage credit risk internally instead, by splitting the pool into tranches so that junior classes absorb losses before senior ones.
The label sits inside a wider statutory family. The Securities Exchange Act defines "asset-backed security" at 15 U.S.C. 78c(a)(79) as a security "collateralized by any type of self-liquidating financial asset (including a loan, a lease, a mortgage, or a secured or unsecured receivable) that allows the holder of the security to receive payments that depend primarily on cash flow from the asset", and lists a collateralized mortgage obligation as one example. A mortgage-backed security is one species of that genus, and a collateralized mortgage obligation is the version that carves the pool's cash flow into tranches with different prepayment exposures rather than paying everyone pro rata.
One classification detail worth knowing before reading any rulebook. Market usage calls agency mortgage-backed securities "agency securities" alongside agency debt, but the regulatory categories are separate: FINRA's definition of an "Agency Debt Security" excludes a securitized product such as an agency mortgage-backed security, which it classifies on its own. The practical consequence is that a rule, a report or a data series about agency debt may not be about these securities at all.
Most people who own them did not choose them. A total bond market index fund holds agency mortgage-backed securities as one of its largest sectors, which is how a retirement account ends up exposed to American refinancing behavior without anyone making a decision about it.
