In addition to Nelda's informative answer, I thought I would answer from the perspective of the kit number. The kit number is an alphanumeric code. Those originally uploaded to GEDmatch classic had a one letter prefix corresponding to the testing site origin (e.g. A=Ancestry, H=MyHeritage, etc.) followed by six numerals. The kits originally uploaded to Genesis begin with two randomly assigned letters followed by seven numerals. Wikitree members add the kit number to their profile (not the raw DNA file).
Pam Tabor