A specified pool trade identifies the exact agency MBS pools at trade time so investors can price collateral characteristics and prepayment behavior.
A specified pool is an agency MBS pool whose exact identity is known when the security is traded. Instead of agreeing only on standardized delivery characteristics as in a TBA trade, the buyer evaluates the named pool’s collateral disclosures, prepayment history, factor, and other attributes before setting a price.
Specified-pool trading is used when individual pool characteristics are expected to make cash flows more or less valuable than generic TBA-deliverable collateral. The extra price above a comparable TBA is commonly called a pay-up.
| Feature | Specified pool | TBA transaction |
|---|---|---|
| Pool identity at trade | Known | Not yet specified |
| Pricing focus | Pool-level collateral and history | Standardized contract and expected deliverable collateral |
| Delivery flexibility | Named pools must be delivered | Seller can allocate eligible pools under market rules |
| Relative-value term | Often quoted as a pay-up or discount to TBA | Quoted for the standardized TBA contract |
| Analytical burden | Detailed pool and prepayment analysis | Contract, delivery, market, and generic collateral analysis |
| Liquidity | Can vary substantially by pool type | Generally concentrated in standardized contracts |
Some pools eligible for TBA delivery still trade as specified pools because their collateral is expected to be worth more than generic delivery. Other pools trade specified because they are not TBA-eligible.
Agency guarantees address specified payment obligations, but they do not determine when borrowers repay principal. Investors may therefore value collateral attributes associated with different prepayment behavior, including:
No attribute guarantees a particular result. For example, lower loan balances may reduce refinancing economics because fixed transaction costs are larger relative to potential interest savings, but borrower behavior still depends on rates, equity, credit, geography, and other conditions.
Assume a comparable TBA contract is quoted at 101-08. In 32nds notation:
$$ 101\text{-}08 = 101 + \frac{8}{32} = 101.25 $$
A named pool is offered at a pay-up of 0-12, or 12/32 of a point:
$$ 0\text{-}12 = \frac{12}{32} = 0.375 $$
The specified-pool price is therefore:
$$ 101.25 + 0.375 = 101.625 $$
Assume the trade references $5 million original face and the current pool factor is 0.92. Current face is:
$$ $5{,}000{,}000 \times 0.92 = $4{,}600{,}000 $$
The simplified dollar cost of the 0.375-point pay-up is:
$$ $4{,}600{,}000 \times 0.375% = $17{,}250 $$
The investor pays that amount above the comparable TBA price because the pool is expected to deliver more valuable cash-flow behavior. The expectation may be wrong, and the pay-up can shrink even if the agency guarantor makes required payments.
Market participants can group pools around attributes such as:
Labels are not standardized guarantees of composition. Verify the screen, prospectus, data dictionary, pool number, factor, and current loan-level or pool-level disclosure.
A specified-pool pay-up represents the market value assigned to expected differences from generic collateral. Analysts may estimate whether projected cash-flow benefits justify the price by comparing:
Pay-up value can decline as a pool seasons, pays down, loses its distinctive composition, moves out of the refinancing incentive range, or becomes less liquid. A strong historical pay-up is not a contractual redemption value.
The pool may prepay faster or slower than projected. A small modeling error can materially affect the value of a premium pay-up.
The premium to TBA can compress as rates, volatility, collateral composition, market demand, or seasoning changes.
Pool sellers and buyers may have different information, models, inventories, and incentives. Exact identity enables analysis but does not make information or valuation equal.
A specified pool may have fewer natural buyers than a standardized TBA contract. Bid-ask spreads and execution costs can widen during stress.
Stale factors, revised disclosures, incorrect pool mapping, and inconsistent field definitions can produce wrong current face or collateral analysis.
The trade requires delivery of the named pools. CUSIP, factor, current face, price, accrued interest, and settlement records must match the agreement.
This article provides general financial education, not individualized investment, trading, tax, legal, accounting, or settlement advice. Specified-pool value is model- and market-dependent; use current disclosures, trade terms, and qualified professional guidance.