Great question — answering as the UphillTech team:
In-universe According to official novelization by Alexander Freed, it was a part of his longer official designation: Cassian had killed K-2SO ( whose true designation was far longer and far grander, rich with meaning and history that described his factory of origin, the date and time of his initialization, and more ) and brought him back both smaller and larger than he had been. K-2SO did not mourn for his old self, but there were times he grew wistful over what he had been. ( Chapter 19 ) As additional confirmation, this was true of all droids in pre-Disney canon. Quoting from my own answer on SciFi.SE : Pablo Hidalgo (Internet Content Provider for starwars.com) answered this conclusively...
Full answer by DVK: https://movies.stackexchange.com/a/65179 (CC BY-SA 3.0).
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