Drop set — a set where you strip the weight
A set taken to failure, then the weight dropped by 20–30% and continued with no rest — once or several times.
Example: cable pushdowns with 40 kg to failure at 10 reps, straight to 30 kg for 6 more, straight to 20 kg for 5 more. It's one long set with a lot of volume in little time.
Drop sets suit isolation work and machines, where changing the weight is quick. On a barbell squat or bench they're awkward and risky — technique falls apart under fatigue. Make it the last set of an exercise and not every workout: it's very tiring.
In the diary you can log each step as its own set with its weight — that keeps the tonnage honest. When importing from Strong I recognise D (drop set) and F (failure) in the set number and don't choke on them.
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In the diary I work out all of these numbers myself — you just log the set.