These are up to 256th order finite impulse response filters, using a polyphase implementation and divided into several versions due to speed concerns: FIR, FIR2 and FIR_FS. The exception is FIRx, which can be up to 1024. They aren't filters in the real meaning of the word, more likely a group of functions with an underlying finite impulse response build but, for the sake of keeping in tone with the rest of the manual, they will be named "filters".
In .TRAN, there's the same causality effect as in the IIRs.

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