Line-level
Cable HF rolloff
Cable capacitance plus source impedance can gently roll off the top treble.
Simple
A long analog interconnect acts like a tiny tone control, very gently rolling off the highest treble — but only if the cable is long and the source is "weak." For normal lengths it does nothing you can hear.
The theory
Every cable has capacitance, and that capacitance forms a low-pass filter with the source's output impedance: f = 1 / (2π · Zout · C_total). The corner frequency is where treble starts to drop. The engine computes it from the cable's capacitance-per-meter × length against the source impedance: above ~100 kHz it's electrically neutral, and it only warns once the corner creeps toward the audible band (≤20 kHz).
For experts
The interaction is what matters — neither factor alone is enough. A 100 pF/m cable at 1 m off a 100 Ω source puts the corner near 16 MHz: irrelevant. Stretch it to a 5 m run of high-capacitance cable (some hit 300+ pF/m) off a high-impedance tube output (say 5 kΩ), and the corner can fall to ~20 kHz — now it's a real, if mild, treble shelf. This is the one place the engine models an analog cable having an audible effect, and it does so through physics, not adjectives: the cure is a shorter run or a lower-capacitance cable, never a pricier one. Phono cartridges are the extreme case — there, loading capacitance is part of the cartridge's designed resonance, which is why turntable cable capacitance is specified, not hand-waved.
Corner frequency of the low-pass formed by source impedance and total cable capacitance (per-meter capacitance × length).
First-order rolloff: −3 dB at the corner, −20 dB per decade beyond it.