Sounds like you looking at the kenngruppen. It was added together with two more "random" characters as the first group transmitted and is used to know/confirm what key that is used. For example, if I was going to send an EWG message today - Feb 23 - from the codebook in enigmaworldcodegroup.freeforums.net/thread/55/optional-2016-ewcg-codebook the first group would contain YKO,HQA,UMH or MMO like
Arduino enigma, sorry, guess it exist different procedures, the one I read regarding kenngruppe is users.telenet.be/d.rijmenants/en/enigmaproc.htm "If we take .. Kenngruppen JKM, OGI, NCJ and GLP. In this case, some examples of a correct Buchstabenkenngruppe are FDJKM, KVOGI or QNNCJ."
Yes, seems like it's different procedures, sometimes first 3 letters and sometimes last 3 letters (and I messed up above and put kenngrup in the middle). Since the procedure closest related to this forum sets kenngruppen in the beginning I guess that's the one to promote here so I will do that going forward.