If you look up “UserData” in the index, you’ll see three different UserData properties that apply to a plugin.
Plugin.UserData is probably the most useful. It allows you to store global data that you can access from anywhere in the plugin code. You can use it to share data between external code and the plugin, between instances of the plugin, or between any callback (in a scripted plugin, not all callbacks get a context argument).
Context.UserData allows you to share data between callbacks (except for those callbacks that don’t get a context, like the OnClicked callbacks in a scripted plugin).