Seeking explanation of "individual flight record - pilot" documents

During Korean War, individual flight records for pilots contained data in the "Mission Symbol" column.  What do the codes mean (e.g., "C-6"; "C-10", "C", "S-3" etc.? What do the entries stand for in the "To" and "From" columns (e.g., To: "Seoul" From: "K-10") - where is K-10 and all the other "Ks"? Is it possible to find out where the missions were to locations marked "Classified"?