IN several threads myself and others have mentioned kits. One can easily forget that not everyone has played 2nd ediiton. AFAIK kits were intorduced i 1989 in the Complete FIghters Handbook and they appeared in most of the 2nd ed class based books and later in Skills and Powers.
A kit was kind of like a back ground you chose at level 1 or on character creation. They usually had some sort of mechanical benefit and often had required proficiencies and bonus proficiencies as well.
Sometimes the disadvanatage was purely a mechanical one or was not readily apparent. Example
+2 AC when not wearing armor or light armor.
+2 on reaction rolls vs members of the opposite sex.
The DM gets to make the characters life "interesting". Your one true love that you seduced? Well they may be the BBEGs child/sibling, or may even be the BBEG. Purely DM arbitration there. The hidden downside was also the opportunity cost putting the high scores into the kit to qualify and the AC bonus is only in light armors or nothing. In 2nd ed if you wore full plate and 18 dexterity you got to add your full dex bonus to AC. At higher levels your AC would probably be about the same as a fighter in full plate as you would probably have magical bracers and a ring of protection which in 2nd ed did not stack with the AC bonus from armor.
Anyway that is just one kit. Others ike the noble warrior give you extra starting money and a reaction bonus vs members of the upper classes, but everything you buy costs more. They were not really balanced from a 4th ed PoV but I do not recall any of them being blatantly overpowered either, but I do not remember the Complete Book of Elves (Book of Munchkins) and I remember the bladesinger being mentioned as OP.
Anyway when one considers out of combat options for the fighters in various threads and kits get mentioned that is what they are and the context in which they used.
Fear is the Mind Killer