Leskova From Germany, joined Oct 2003, 6075 posts, RR: 71 Reply 1, posted (8 years 7 months 2 weeks 6 days 1 hour ago) and read 3352 times:
HK is the status code for a confirmed seat - sometimes said as "Holding (or Have) Konfirmed".
TKTL is a "Ticketing Timelimit" which notifies the agent (travel agency, airline, ...) that the ticket needs to be issued - in this case, the reminder was set for 22. April and the reminder was for the office with the code LJUJP0028 - I'm guessing that that's Adria Airways' office at either the airport or the city of Ljubljana.
A SSR is a "Special Service Request" sent to the airline's system - in this case, it's the Frequent Flyer number.
OSI is, if I recall correctly, "Other Service Information" (I always use the term OSI - I never really think about what that's an abbreviation for), OSIs are used for sending all types of information to the airline - everything from Ticket Numbers, sometimes codes for special fares (which I think it'll probably be here), references to other bookings made for passengers travelling together with the one in the current booking, ...