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Idiosyncratic fCash refers to an fCash asset whose maturity does not correspond to an active liquidity pool. After a quarterly roll, an fCash asset whose maturity previously corresponded to an active liquidity pool can become idiosyncratic.
For example, consider a user who took out a loan from the one-year liquidity pool. At the time of the next quarterly roll, that user's fCash would mature in nine months, and the nine-month liquidity pool would reset after the quarterly roll to be a new one-year liquidity pool. This would result in the user's nine-month fCash becoming idiosyncratic.
Idiosyncratic fCash can't be traded on one of Notional's on-chain liquidity pools because an idiosyncratic fCash asset's maturity does not match an active maturity. Notional supports trading idiosyncratic fCash via submission of a transaction that is signed by both the sender and receiver of the fCash asset. Idiosyncratic fCash liquidity can be made available via external orderbook or RFQ-style infrastructure like 0x, but this functionality is not yet active.
For the user who holds nine-month fCash, this means that currently they would not be able to sell their fCash on a Notional liquidity pool for a period of three months until it became six-month fCash and could be traded on the six-month liquidity pool.