Any Object can be registered to the Context as a subsystem, by using the function RegisterSubsystem(). They can then be accessed by any other Object inside the same context by calling GetSubsystem(). Only one instance of each object type can exist as a subsystem.

After Engine initialization, the following subsystems will always exist:

Time: manages frame updates, frame number and elapsed time counting, and controls the frequency of the operating system low-resolution timer.
WorkQueue: executes background tasks in worker threads.
FileSystem: provides directory operations.
Log: provides logging services.
ResourceCache: loads resources and keeps them cached for later access.
Network: provides UDP networking and scene replication.
Input: handles keyboard and mouse input. Will be inactive in headless mode.
UI: the graphical user interface. Will be inactive in headless mode.
Audio: provides sound output. Will be inactive if sound disabled.
Engine: creates the other subsystems and controls the main loop iteration and framerate limiting.

The following subsystems are optional, so GetSubsystem() may return null if they have not been created:

Profiler: Provides hierarchical function execution time measurement using the operating system performance counter. Exists if profiling has been compiled in (configurable from the root CMakeLists.txt)
EventProfiler: Same as Profiler but for events.
Graphics: Manages the application window, the rendering context and resources. Exists if not in headless mode.
Renderer: Renders scenes in 3D and manages rendering quality settings. Exists if not in headless mode.
Script: Provides the AngelScript execution environment. Needs to be created and registered manually.
Console: provides an interactive AngelScript console and log display. Created by calling CreateConsole().
DebugHud: displays rendering mode information and statistics and profiling data. Created by calling CreateDebugHud().
Database: Manages database connections. The build option for the database support needs to be enabled when building the library.

In script, the subsystems are available through the following global properties: time, fileSystem, log, cache, network, input, ui, audio, engine, graphics, renderer, script, console, debugHud, database. Note that WorkQueue and Profiler are not available to script due to their low-level nature.