In simple terms, we track a parcel that comprises some (or part of) particles. The parcel trajectory is determined from all the drag laws etc for a particle of the diameter & density seen in the plot. When source terms (evaporation, erosion etc) are calculated we use the parcel mass to determine the effect. It's termed "parcel theory" and should be covered in some of the theory books: I did write up a good explanation in the Multiphase training course but that's on the Learning Hub and can't be shared here.
The terminology has become very ingrained, and given the TUI reports show trapped/escaped/etc particles as opposed to parcels it's not going to be untangled any time soon.
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