Alignments
Currently a big work in progress
IMPORTANT NOTE: in cases where a real-life context differs from the 2nd Edition D&D Planescape lore, SPL considers the 2e lore context correct in our fantasy setting. This means that a modern-day interpretation of a concept like Mercy or Tyranny may conflict with a published fantasy concept. It is important in Planescape that the 2e locations of concepts and their examples are maintained for story integrity. Much like some suspension of disbelief is needed for eg. devils and demons, so too is it needed for certain concepts.
Alignment in General
Within Dungeons and Dragons, Alignment is a shorthand description of a complex moral code, sketching out the basic attitudes of a person, place, thing or even idea. Alignment is not personality, but alignment does shape personality. It is a moral compass, deciding preferred actions, ideas and guidelines for how a Planeswalker would approach ethical decisions and moral dilemmas. The Player should see Alignment more as a tool for describing their actions, rather than dictating their Character's actions based on Alignment like a ball and chain, nor should it be seen as a punishment for misbehaving Planeswalkers.
A Character's Alignment can change as they evolve and continue to change, or not change, and as they live their lives within the confines of Roleplay. Although a Character's Alignment can be fluid to a degree, it often signifies a great turn of events, or perhaps a great ordeal, a trial, redemption or fall. In turn, a change of one's Alignment also implies the change of not only their attitudes, but also their perception and relation to the world around them.
In practicality, Alignment and how one uses it gives interesting situations from the conflict and cooperation between differences, whether it'd be a party of Good Planeswalkers setting aside their differences to defend a village from an Evil party of Planeswalkers, a Lawful group squabbling over minutiae of how something ought to be shared after a long cycle of planeswalking, or an Evil cabal of wizards cooperating while slowly stabbing eachothers' backs, etc.
However, there is a big difference between general D&D and Planescape...
Alignment in Planescape
Unlike in regular D&D, Alignment is one of the central themes of Planescape. It not only represents the behaviour of regular mortals on Primes, they are not only philosophies, but actual Forces of the Cosmos, manifested within the Outer Planes. Those Planes are shifted and moulded by the mass collective belief of every living mind, like a sea of belief.
Reified belief exists as infinite planes, as outsiders, as strange immaterial forces. They aid and hinder, kill and heal, empower and weaken. Entire infinite planes exist within the purest ideal of their respective Alignment, each showing how their way is when at their strongest. Law, Chaos, Good and Evil even manifests into outsiders, beings of pure belief-turned-substance who live, breathe and die in, to a normal mortal, alien ways entirely devoted to their (im)moral beliefs. Great forces clash both in mind and steel to show what way is the best, such as the Blood War, an infinite battle of which sort of Evil is better (though other alignments dabble within the conflict).
While there are many philosophies, belief systems, deities, demigods and so forth, the thing uniting them is on two axes, their adherence to Law and Chaos, and their adherence to Good and Evil. Good is always opposed to Evil, and Chaos is always opposed to Law.
It is also very important to note that within Planescape, actions can be deemed objectively good or evil, as every good or evil act resonates with the cosmic forces of Good and Evil. For lawful and chaotic actions, this is a lot simpler, as it is very hard for one to relativise lawful or chaotic actions.
A Note on Planescape's Concept of Alignment
Surrounded by such extreme examples of Alignment, a Player might think that their Character will be extremely lonely, one should remember that Alignment serves to show both similarities, but also differences. As an example, Lawful Evil and Lawful Good characters might work together in a specific Lawful concept or mission, but under strenuous circumstances. Some classes or concepts might greatly struggle in Planescape as they adjust to the sheer cosmic battles which the Outer Planes are, but that is not to be seen as a negative, but rather an opportunity for growth against adversity. Additionally, unless you're roleplaying one of the more difficult classes which are tied to cosmic forces of Good and Evil, or are roleplaying an Outsider, you are allowed to move and shift from the absolute notion of your Alignment; eg. a character can be Lawful Good without being utterly like Mount Celestia (but greatly deviating from those concepts might warrant an alignment change). Additionally, due to how alien Outsiders are, mortals might find some Outsider behavior strange or alarming, but that is simply due to the great differences of how an Outsider (pure belief) thinks, versus a living mortal.
The Nine Alignments
Due to the sheer number of Alignments, each of the nine Alignments has its own page, describing it in detail, the examples of behavior, a society of its alignment, as well as the outsiders and Outer Planes representing it. Additionally, all of the four forces have their own page for more information. The information within is not comprehensive and absolute, but should be taken into account when considering one's Character and their Alignment.