It stands for super principal only tranche; a tranche that constitutes a special class of principal only (PO) tranches embedded in a securitized product (e.g., CMOs) with a PAC or TAC tranche. The PAC or TAC tranche has the effect that all the call risk associated with the underlying collateral is transferred/ diverted to the other tranches (in which case, CMO tranches) in the structure. A super PO tranche is a zero-coupon support tranche for PAC or TAC tranches in a collateralized mortgage obligation.
With the presence of a super PO tranche, all generic or ordinary PO tranches become very much sensitive (hyper-sensitive) to potential calls (call risk) by a mortgagor. The hyper-sensitivity renders a principal only tranche into a super principal only tranche.
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