An Orbital Defense Platform is simply "platform" is a fixed weapon emplacement usually placed on langrangian orbit. They can vary tremendously in size and armament and are used in many kinds of engagements, they almost always exclusively as defensive weapons.
Platforms are defines as being separate from ships in that they are unpowered. Some propulsion is required for station-keeping, and several designs could travel at extremely low speeds. Old battleships were also frequently repurposed as weapons platforms.
Since the mid Succession Wars, the designs have taken on a strictly modular approach, being separated into power, propulsion, missile, and directed modules. A smaller combination power-propulsion module was also produced. In the cases were these platforms were deployed. Typically, one power module was required in order to operate one directed energy module, which would also require a propulsion module. Typically, at least one missile module could also be attached without over-taxing the design.
The simplest platform involved a single combination module coupled with two missile modules. More complex platforms would sometimes link up hundreds of modules and add additional redundant power supplies, providing a defensive system with the firepower of a capitol ship.
Old warships frequently converted to act in this function. Capitol-class warships were often forced into retirement by the lifespan of their engines. In war, the propulsion systems were typically pushed dramatically beyond design limitations, and catastrophic failures could occur. If the damage was bad enough that the ship could not be taken in for refitting, and the hull was not new enough to warrant a tow, it would be placed in a parking orbit and sued as a fixed weapons platform. These vessels, acting both as weapons systems and deterrents, were so effective that the Foundation began actively building and deploying large-scale platforms designed to look like capitol ships.
The Alliance is far and away the largest operator of this technology, with defense satellites out-numbering warships seven-to-one. Automated missile pods and drone-control systems are the most common, though larger platforms are also frequent. Large-scale platforms, some the size of small capitol ships, out-number those same ships by four-to-one.
In contrast to the Foundation, the bulk of Alliance platforms are manned installations, with larger platforms being the norm. The smaller, automated or remote systems are much more plentiful, this is owing only to their inexpensive production costs. Typically, a handful of large, manned stations, controlling dozens of smaller unmanned systems, will form the basis of a planetary defense grid.
The reasons for this highproponderence of fixed platforms is owed to Alliance law. When the modern iteration of the Alliance was formed at the end of the Second Age, they laid down a series of laws regarding shared military assets for the common defense.
The laws in the charter specified a number of guns that had to be deployed, not a number of ships. Thus, the terminally under-funded UESAF fell to defense platforms as a way to meet the unrealistic requirements.