Kits were a thing in 2e, here is a pretty good thread on them. The first couple posts in that link do a good job of explaining them without getting into the crunch of 2e itself. those are almost certainly the kits in questionthis means like herbalism kit or disguse kit right?
Kits were originally introduced in Complete Fighter and were designed to be a way to further customize characters to a specific concept, without having to create whole new classes. So you'd pick a class and then pick a kit from the list of kits for that class - a fighter could be an Amazon or a Beastrider or a Barbarian or a Savage, while a wizard could be a Savage Wizard or a Militant Wizard or a Noble Wizard, and a thief could be a Smuggler or an Assassin or a Troubleshooter.
Each was a package of abilities - typically one special benefit, one special hindrance, a couple of bonus Non Weapon Proficiencies, and often a recommended weapon. Quite often there are reaction roll modifiers to other characters.
The kits do vary greatly from just being cool concepts, to pretty over the top: some of the dodgier ones include the elven Bladesinger kit, the bard Blade and the 3-armed tree ranger from the Ranger's Handbook. Some of the fancier classes (particularly Bard) had kits which radically redesign the class; a bard kit is basically a wholly different type of bard and loses all the standard bard powers for a completely different list of special abilities - like a blade (guy who shows off weapons skills) getting defensive spin and ambidexterity, instead of the normal song abilities. A number of the kits allowed unusual races to pick up a class, as long as they took a kit e.g. dwarves could be Bards if they took the Chanter kit, and halflings could be Ranger/Explorers.
Kits were initially largely for single-classed characters; the race handbook's however include a number of kits available to multiclassed characters. Skills & Powers had a list of rebalanced kits which were available to all character classes (and regardless of multiclassing) as well. Planescape noted that only 'Prime' characters had kits - they got these instead of choosing a Faction.
One of the design decisions that went into 2e was the insistence on keeping the number of actual classes small, usually limiting them to just those found in the PH. Kits were a way of compensating for that self-imposed limitation by giving people specific builds to customize those basic classes. Therefore instead of having a Barbarian class, you could apply a barbarian-flavored kit to the Fighter class, or so the logic went. Many discrete classes from 1e became kits in 2e, added to whichever basic class they were seen to be most like. The 3e designers reversed that trend and made them either full classes or prestige classes, leaving kits a uniquely 2e thing.
In theory, a kit's additional benefits were supposed to be counterbalanced by specific drawbacks so that adding a kit would not make the class objectively better than the base class. In practice the authors of 2e were only loosely acquainted with balance as a concept, they often got carried away with the coolness of a given concept, and power creep was a thing. As a result, you were often better off taking a kit than not.
In a way they would probably be very useful for setting localization type things that could be applied blanketed across the classes without having the kind of pushback from players that tends to come with "x class is not allowed" even if nobody planned to play x.