If we scan the books of Mirza Qadiani, we find the following qoutes from his books which clearly highlight what did Mirza understand by this word 'KHATAM':
Tunapoviangalia vitabu vya Mirza Qadian, tunakutana na nukuu ambazo zinaweka wazi jinsi Mirza alivyoilewa maana ya Neno Khatam: