43X is inbetween Gate 43 and 45 in T4. Looking at Bing Maps in Bird View it seems that it is used for larger aircraft. But with the angle it seems both Gate 43 and 45 cannot be used if 43X has an aircraft. The jetway for Gate 43 looks like it can be used for 43X. So I would assume that you would just use the Gate 43 ticket counter and jetway.