Load-time is when mod code is first run, files are actually run at the top-level.
Run-time is when the game loop is running, code inside registered functions will be run.
Nodes can only be registered at load-time.
This is because they are closely linked with the engine and it's map format.
Chat-commands can be registered whenever.
They are entirely client-side, this is why we don't have auto-complete and the sorts.