[wrl]

The short answer is that there really aren't. All extant audio plugin APIs/formats are basically ways of getting audio data into and out of a `process()` (sometimes called `Render`) function which gets called by the host application whenever it needs more audio from the plugin.

Every API has its own pageantry not just around the details of calling `process()`, but also exposing and manipulating things like parameters, bus configuration, state management, MIDI/note i/o, etc. There are differences in all of these (sometimes big differences), but there aren't any real crazy outliers.

At the end of the day, a plugin instance is a sort of "object", right? And the host calls methods on it. How the calls look varies considerably:

VST2 prescribes a big `switch()` statement in a "dispatcher" function, with different constants for the "command" (or "method", more or less). VST3 uses COM-like vtables and `QueryInterface`. CLAP uses a mechanism where a plugin (or host) is queried for an "extension" (identified by a string), and the query returns a const pointer to a vtable (or NULL if the host/plugin doesn't support the extension). AudioUnits has some spandrels of the old mac "Component Manager", including a `Lookup` method for mapping constants to function pointers (kind of similar to VST2 except it returns a function rather than dispatching to it directly), and then AU also has a "property" system with getters and setters for things like bus configuration, saving/loading state, parameter metadata, etc.

I'm not sure why OP is claiming that AU is somehow unopinionated or less limited. It doesn't support any particular extensibility that the other formats don't too.

[dmix]

Is there some software to convert VST3 to AU? Or do you develop both separately?

Someone else said

> Almost all VST plugins have an AU version (like 80%-90% or so, and 99% of the major ones).

Which I noticed as well, I wondered if that required a large time investment to support both or there's some API translation layers available.

[wrl]

Historically, there was an API translation layer that VST2 plugins used (called Symbiosis), but these days the vast majority of plugin devs use a framework like JUCE which has multiple different "client-side" API implementations (of VST2, and VST3, and etc) that all wrap around JUCE's native class hierarchy.

There's a few other frameworks floating around (Dplug for writing in D, a few others in C++), but JUCE is far and away the most common.