Plugins
Plugins are one of the main ways to customize Player to suite individual use-cases. Internally they allow access to many of the core sub-systems, which can add features, configuration, or custom behaviors.
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The scope of what a plugin is capable of is pretty broad, but are typically broken down into smaller reusable modules. Some are more end-user focused (Common Expression Plugin and Common Types Plugin) while others are more relavant for other plugin developers (Expression Plugin and Types Provider Plugin)