In a pass-through structure, each certificate holder will be allotted a proportion of the cash flow from the underlying pool of loans or receivables on a pro rata basis. Suppose an asset-backed security has the following structure:
Senior tranche Rs.100 crore (1000 certificates issued),
Subordinated tranche Rs.10 crore (100 certificates issued).
In the above structure, each certificate holder of the senior tranche and subordinated tranche would receive 1/1000 and 1/100 of the cash flow to be paid to the senior tranche/subordinated tranche from the collateral respectively.
In a pay-through structure, while a senior tranche can be split into different tranches, a subordinated tranche cannot be done so. The combined par value of the components of the senior tranche will be equal to the par value of the original senior tranche.
Let us recall that the senior-subordinated structure is useful to minimize the credit risk. This is because the loss in the deal can be met from the funds in the subordinated tranche. Thus, credit risk is redistributed from the senior tranche to the subordinated tranches. This process is referred to as credit tranching. If the senior tranche is split-up into different tranches each with different exposure to prepayment risk in a pay-through structure, prepayment risk is redistributed among the components of the senior tranches. This process is referred to as prepayment tranching or time tranching.