designated preliminary spots are spots for people going into subspecialties. i.e.
ENT, Ortho, Urology, Neurosurgery all require 1 year of internship in general surgery. These are designated preliminary spots because they are only for people going into these subspeciallities
Integrated plastics programs also require 3 years of prelim gen surg these are also designated preliminary spots.
Non designated spots are available for anyone to apply to. Usually these are for people who did not match or for people whose subspecialty programs do not have designated prelim spots for them.
Some smart radiology residents will also apply for non-designated prelim spots in surgery so that they will experience the life and philosophy of the future customers (surgeons).
Also some programs will not give categorical spots to FMG's and so the FMG's take prelim spots in hopes of doing so well that they get converted to categorical (if a spot opens up).
Finally, the RRC (residency review committee) only alows surgical residents to maintain the prelim classification upto the PGY-3 year then these non-designated prelims either need to get categorical spots somewhere to continue in surg (they can get PGY-1 through PGY-4 spots) or they need to find another specialty.