>>945
The seemingly random words the Machine uses to communicate social security numbers
correspond to the Dewey Decimal System (DDS) numbers on books in Finch's library.

The words themselves are individual codes leading Finch and Reese to the correct books:
the first word (Uncertainty) is the title keyword, and the second and third words
(Alpha/Mike, from the NATO phonetic alphabet) correspond to the initials of the writer.
With three books side-by-side, the DDS numbers form the social security number of the person of interest.

https://personofinterest.fandom.com/wiki/The_Contingency