# SubsystemRegistration

> Callback invoked when starting up the runtime. Called before the first scene is loaded.

## Definition

* **Type:** Field
* **Namespace:** [UnityEngine](/engine/6000.6/script-reference/unityengine.md)
* **Assembly:** UnityEngine.CoreModule

```csharp
SubsystemRegistration = 4
```

### Remarks

For new code, use [OnCodeInitializingAttribute](/engine/6000.6/script-reference/unity/scripting/lifecyclemanagement/oncodeinitializingattribute.md) instead for better resource pre-allocation timing. For more information, refer to [Code reload and the code lifecycle](/engine/6000.6/manual/scripting/compilation-and-code-reload/programming-code-lifecycle.md) in the manual.

For more information on the execution order of this option relative to the others, refer to [RuntimeInitializeOnLoadMethodAttribute](/engine/6000.6/script-reference/unityengine/runtimeinitializeonloadmethodattribute.md).