In which situation the fee rate can be negative and what does it really mean?

Here in the definition of CFeeRate class is a member called nSatoshisPerK:

/** * Fee rate in satoshis per kilovirtualbyte: CAmount / kvB */
class CFeeRate
{
private: /** Fee rate in sat/kvB (satoshis per 1000 virtualbytes) */ CAmount nSatoshisPerK;

And its job is obvious according to the comment above. This class has also a member called GetFee:

CAmount CFeeRate::GetFee(uint32_t num_bytes) const
{ const int64_t nSize{num_bytes}; // Be explicit that we're converting from a double to int64_t (CAmount) here. // We've previously had issues with the silent double->int64_t conversion. CAmount nFee{static_cast<CAmount>(std::ceil(nSatoshisPerK * nSize / 1000.0))}; if (nFee == 0 && nSize != 0) { if (nSatoshisPerK > 0) nFee = CAmount(1); if (nSatoshisPerK < 0) nFee = CAmount(-1); } return nFee;
}

My question is that in which case it is possible for the nSatoshisPerK to be less than zero? and what does it mean?

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