Every QKD datasheet opens with a headline key rate and a maximum distance. Neither number means much without the conditions attached to it, and the conditions are usually in a footnote.
Secret key rate
This is the rate at which the system produces key material that survives error correction and privacy amplification. It is not the raw detection rate, and the gap between the two is large. The question to ask is at what distance and at what loss the quoted rate was measured, and whether the measurement ran long enough for the finite-key corrections to be honest.
Why duration matters
Security proofs are asymptotic in the limit of infinite key material. Real systems produce finite blocks, and the correction for that is not small. A rate quoted over a ten minute run and a rate quoted over a week are different quantities.
Quantum bit error rate
QBER is the fraction of the sifted key where the two ends disagree. It rises with loss, with noise from classical channels sharing the fibre, and with anything that disturbs polarisation or timing. Above a threshold, no secret key can be extracted at all. A system that holds a low QBER on spooled fibre in a laboratory and one that holds it on deployed fibre through a summer are not comparable.
Maximum distance
Distance is a proxy for loss, and the relationship depends on the fibre. A specification given in decibels of channel loss is more useful than one given in kilometres, because it transfers directly to a route you can measure.
The number that is usually missing
Availability. A link that produces an excellent key rate for six days and stops on the seventh is worse for an operator than a modest rate that never stops. Ask for continuous operation figures over months, with the interruptions included rather than excluded.