Despite the widespread use of VST, AU and AAX, there are effects and instruments in other formats on the Internet – RTAS, DXi and ReFill. The first two are legacy formats, the third is intended exclusively for DAW Reason from Reason Studios.
The RTAS plug-in format was created by Avid Technologies in response to the emergence of the VST plug-in format in the music market. The developers decided to drop VST support, creating their own exclusive format. Real-Time Audio Suite – this is how the full name of the plug-ins of this format sounds. It was developed specifically for Pro Tools, although this did not generate much enthusiasm among the users of this program.
Along with the release of VST2, Microsoft entered the format race, which together with Cakewalk created the DXi (DirectX Plugin) format. Aside from the fact that the format relies on DirectX technology, DX plug-ins work the same way as VST, AU, and AAX. The idea behind DX plug-ins is that the use of DirectX libraries allows processing to interact not only with DAW, but in general with any sound program in the Windows environment. This allows DX plugins to connect to audio and video players, video editors and other software. To regular DAWs, DX plug-ins look like regular VST processing.
At some point, Propellerhead spun off from the VST design and enhancement project, focusing on developing its own Reason ReFill format, compatible only with DAW Reason. How ReFill works is a sound library that combines samples, MIDI files, grooves, patches, and even entire song projects. ReFill plug-ins can work as a virtual instrument with its own set of effects, like a regular emulation of a real analog device or a set of samples. The format is for DAW Reason only and is not suitable for use with other music recording software. In 2019, Reason Studios stopped active development of ReFill and implemented VST support in Reason. Moreover, now the DAW Reason itself can work as a plug-in in other DAWs, and the ReFill effects and plug-ins from the Reason composition are available to users in any program.
SFZ (abbreviated Sforzando) is another highly specialized format for storing musical information. The format is a standard for defining the behavior of virtual musical instruments and defines how libraries and effects will sound. Thus, SFZ offers an alternative to NKI and allows the creation of virtual libraries.
As far as NKS is concerned, NKS (Native Kontrol Standard) is not a full-fledged plugin format. NKS technology enables deeper integration of hardware controllers (such as MIDI keyboards) with NKI virtual libraries and VST / AU plug-ins. The NKS icon only denotes support for integration from the side of plugins and hardware, however, NKS itself is not a stand-alone format.