Binding Principle

The creation of constructs and the necromantic reanimation of dead bodies operate on similar mechanisms. Both involve using non-vital energy to give a semblance of life to that which is by nature inanimate, and are thus by definition unnatural. Both achieve their effect by applying a mystical formula to a physical vessel, then charging that formula with energy, although the energies and exact formulae differ. It is primarily the use of that which formerly lived which makes automata a subject of curiosity, and undead a target of loathing.
An animated thing, construct or undead, acts according to the dictates of the formula that animates it. The more complex the formula, the greater the discretion the construct or undead can act with. The source of its power limits how long it can remain active, and how strongly it can act to affect the world. Without an internal source of power, such as a soul or forgeborn core, any construct or undead will only persist until the energy poured into the formula is expended. Constructs tend to stop when out of power, but undead draw a thread of necrosis from the Grim Plain as part of their nature. Instead of running out of energy, the necrosis tends to erode their binding principles, at which point they either collapse or simply become independent, and eventually to decay to nothing.
The simplest forms of construct are called apparati, animated objects designed to perform a range of simple actions. The most rudimentary simple undead use a similar formula, but bind the energy into a once-living body as vessel. An apparatus's formula is called a coda, and is very similar to the workings used without a vessel to create unseen servants, or even a basic mage hand. A coda contains instructions to do one or two things, either when commanded to do so, or at any given opportunity.
A rubrik is an exponentially more complex formula, used to create servitor-level constructs, including golems and more flexible undead servants. Such servitors cannot be created quickly, requiring multiple complex procedures to lay the rubrik into many different levels of physical or spiritual material. A rubrik allows the construct to do a number of things - usually half-a-dozen related actions - and to assess when best to do each of those things.
The most advanced constructs are based on the titanic forgeborn proxies. The animating matrix of a forgeburn is rooted in their core but extends throughout their body, and is a further exponential expansion of a rubrik. A forgeborn core is also a power source, converting a mix of divine and arcane energy into animating force. An undead being of similar complexity would be a terrible thing, not least because it would require a power source capable of combining divine and necrotic energies. A matrix contains a sufficient corpus of recorded experience, conditional instructions and ethical imperatives for the construct to act at least with an approximation of free will and sentience. It also contains the capacity for learning new skills.
A similar effect can be achieved by binding a soul into a vessel as both power source and animating formula. This is used for creating sentient artefacts and certain powerful undead, but always carries a certain danger of a billion. The binding in such instances does not always require a deliberate act of magic, but can be caused by a curse, although with curses it often occurs that a fragment of the soul is instead caught, acting like a rubrik or coda.
A bound soul or the highest order of forgeborn matrix is actually capable of acting with free will, and possesses sentience. By its nature, a bound soul cannot, however, learn new things, although a forgeborn matrix can.