Fixed term tranches have an automatic maturity period after which they become claims on the underlying asset.
Perpetual tranches have no maturity period. As a result they can be used to resegment volatility indefinitely.
Tranching is the process of resegmenting the volatility of an underlying asset into two or more different assets with different volatility profiles. In the SPOT protcol, tranches are represented by ERC-20 tokens.