It seems strange and counterintuitive that one kata could make up a complete fighting system. How could one kata, which usually takes only a couple of minutes to perform in it's entirety, be a complete fighting system? It's really not that strange. Think of how many popular songs there are that only contain three to five chords. There are thousands. It's the space between the notes and not the notes themselves that make music. Kata is very similar. It is only limited by creativity, physics, anatomy and physiology. It is the instrument, we are the musician.
In order to look at each kata as a complete system we need to change our point of view a little bit. Shift our perspective. Many times we are taught that kata is a collection of techniques designed as defenses for specific attacks. Even if we don't hold to the view that a kata is designed for fighting ten men at once, analysis can be rather specific. He grabs my wrist, therefore, I perform this section of the kata, which is the wrist grab portion. This is too much of a micro view, not to mention being tactically wrong. We need to take a very generalized view and apply the kata techniques to the variables that we are almost 100% sure will be present.
Some things we can be almost positive of in a conflict: the enemy will have two arms and legs, they will have two eyes, they will have functional hearing, their balance is dependent on their body alignment and structure, their joints are weaker than their bones, their neck is weaker than their skull, your hand is stronger than their fingers, your arm is stronger than their hand, and your two arms are stronger than their one arm. They will also be limited in their planes of attack. They can either come from the front, behind, to either side, and the attacks generally come either medially (down the center) or laterally (coming in from the sides). The opponent will strive to work against you.
There will of course always be exceptions, but in general we can expect these things. These are variables we can prepare for and are the only things we can know for sure. Everything else is luck and unpredictable, so there's no point worrying about it.
Kata techniques are not defensive in nature, they are offensive with a defensive component. Much like keeping your guard up while punching. They are attack and defense together. Being able to defend against specific attacks accomplishes very little. Blocking an attack doesn't stop the attacker, it only stops that single attack. You want to stop the attacker, which will end all attacks, not just individual ones. This is why all kata movements should disable an attacker immediately. Let's go back to the wrist grab example. Escaping from the wrist grab solves very little. It doesn't stop the attacker. Jabbing them in the throat, or slapping an ear with a cupped hand will usually stop the attacker and allow you to free your wrist. It can also stop another attack in progress while you do it, if done correctly. It's also easier to remember and perform than a multiple step wrist extraction.
The techniques are also the embodiment of principles. If the principles are used, the shape of the technique can change, but it will still work, because it's built on those principles. Think of an architect. If he understands the principles of construction, he can design any structure he wants as long as it adheres to those principles. He can build a shack or he can build a cathedral. One must understand the principles of a kata, so they are not bound by it's form, but only by it's function. This is also how you get a thousand interpretations of one kata movement that all seem plausible. It's because they are the same just different expressions. Like music, same instrument, different songs.
Strategy is what links all of these techniques together. They compliment and build on one another to achieve a goal. It gives you a clear end to work toward, and you can ignore all other variables that don't get you closer to that goal. This is also the biggest difference between all of the kata, strategy. The strategy of one kata is different than the other. The version of Seisan kata that I practice has the strategic goal of disrupt, unbalance and knock down violently. Basically to put them on their butt, so I can make a hasty retreat, how badly I hurt them to achieve this depends on the situation. The strategy of another kata may be to break the opponent's neck and all the techniques focus on getting you to that end goal. Think of the difference between boxing and judo. In boxing the strategy is to pummel the person with your fists. In judo, it's to throw them into the ground. If you tried to throw a person by using boxing maneuvers you'd fail, just like if you tried to strike a person with judo maneuvers. Each has a specific strategy and their techniques work towards that strategic goal. You can't use one to do the other, at least not optimally.
Let's add up these different components. A kata is a collection of principles, embodied in movement, designed to incapacitate a person, while keeping yourself protected and linked by a common strategy based on the variables that we can be most sure will be present. In other words, a fighting system.