In essence, the AI Core is the component that stores the conscience of a sentient machine. Its exact capability, form and appearance varies by origin. They possess great computing power, but their intelligence or problem-solving ability is not greater than a human's, except when it comes to doing complex mathematical calculations - a skill necessary for and focused around their targeting mechanisms. Though they are able to feel emotions, they tend to be somewhat lacking in emotional intelligence. As their memory space is not infinite of course, they eventually forget things like humans do, to make space for new memories and skills. It is also necessary for them to sleep, if usually less than humans do at a time, but more frequently. They get tired - the function of that is to remind them to sleep regularly. During sleeping, their cores enter a more energy-saving mode and defragment themselves, which may result in strange dreams.
Classes and Roles
While most of their body can be changed around them, vehicles can't change their core. It can adapt to new bodies with remarkable versatility, but there seem to be certain, apparently random talents and shortcomings to each individual core. This manifests, for example, in preferences for particular fighting styles. Based on this, the vehicle fulfills different "roles" in battle and performs best if it is used in the way that its core functions. Examples for roles would be brawlers or scouts. Generally, these behaviour patterns can be observed in a large number of vehicles, which leads to them being sorted into the so-called "classes". Examples for classes are the Light Tanks, or Tank Destroyers. Sometimes, a vehicle knows right away what its class is, but this initial guess may be wrong; and usually, the journey to discover one's class will be a long one. When talking about classes, a vehicle may also be referring to one's current body type, but confusion of the two meanings is almost always prevented through context. While most factories are known to produce vehicles of most or all classes, there are also a few that only produce one class, or alternate production in batches. Most tank classes are distributed evenly, although Main Battle Tanks are the rarest among them. These tanks are already produced in their final body, which simplifies the finding of their class greatly. This doesn't mean that other tank classes cannot use MBT parts/bodies, but they will have greater difficulty doing so.
As opposed to the body, maintaining a core is quite difficult. Malfunctions are hard to pinpoint and fix - in most cases the primitive surgical methods used cause more harm than good. This circumstance gave rise to the mindsmiths, a subgroup of TRVs who have made it their trade to attempt to fix or at least alleviate the problems with the damaged or dysfunctional core. Their methods are usually fruitless, but there have been known cases where therapy was able to improve lives. As few things last forever, cores will eventually start degrading over the years. Common signs of this is memory loss in varying severity, to the point of constant anmesia, occasional paranoia, or loss of coherent speech, and marked loss of drive accompanied by overwhelming drowsiness. Unlike the rest of the body, the technology to extend the life of a core is woefully underveloped. It doesn't help that messing with a vehicle's "brain" is considered an ethic taboo.
Life and Death of a Core
Cores are built by factories, using a procedure that the vehicles don't have the technology for to synthesize. Tank cores are always built into tanks, plane cores are always built into planes, and so on.
Keeping it alive
A core needs a constant energy supply, which is electricity. In all vehicles, this electricity is supplied by batteries. To keep these batteries powered, a vehicle must either be hooked up to a direct electrical outlet, or keep their engine running. Ships are an exception - as a third option, their sun collectors allow them to passively collect and store the electricity their cores require. When a core is cut off from electricity for even short times, memory loss can occur. Its severity increases exponentionally the longer the core is without. Eventually, it will result in a total wipe; effectively, a bricked core that can't be restored with the current technical means. Cores are also quite sensitive to heat, and may shut down when the insides of a vehicle reach a certain temperature. Under normal circumstances, it can cool itself down enough to survive hot weather with the help of the engine's liquid cooling cycle and the vehicle's ventilation. When this mechanism gets damaged however, even average temperature may lead to severe core damage.
Killing the Vehicle
Hitting the core in battle is a safe and easy way to take out an enemy. A direct hit or even a glancing blow can either fully destroy or severely damage the core. The results range from the victim being killed, to the victim losing a great part of their core's functionality; sometimes to the point of complete dependance on others. As a counter-measure, vehicles frequently relocate their cores to a new position to minimalize the chances of it getting hit. Popular - and thus, unfortunately, easy to guess - hiding places are the most heavily armoured parts of the hull or turret. This is a time-consuming process, and therefore not done during battles.
AI cores may be defect upon construction, or suffer from various dysfunctions that can develop throughout a machine's life. Generally, poor availability of "healthcare" and a constant shortage of supplies means that most dysfunctions are disregarded or concealed from others until the symptoms get too severe to ignore. Even if an illness is spotted earlier, the means to fix it - if possible to be fixed at all - are usually not there. Several problems arise because of this situation. The fact that in most cases, illnesses only get recognized once something extreme happened leads to strong stigma for sufferers. The general populace is poorly educated on symptoms and early warning signs, which leads to self-proclaimed experts diagnosing healthy individuals with all sorts of illnesses and causing serious trouble for them. Consequences for sufferers if their illness is spotted are unhelpful at best, and fatal at worst. Whether they are a danger to themselves or others or no one makes little difference; every dysfunction, perceived or real, is seen as a threat to their surroundings. Actual sufferers are therefore hesitant to get help if they suspect they might be affected. Well known and feared illnesses include the Feind Syndrome and Mindrot.