A plug-in standard included by Apple in its OSX operating system. Audio Units (AUs) is intended to replace manufacturer-specific plug-in standards such as VST or MAS on Apple systems. In theory, all sequencer software and plug-ins written for OSX will support this standard, and plug-in authors will no longer have to write multiple versions for different standards. In current practice, most Macintosh plug-ins currently are offered in both AU and at least one other format, usually VST or RTAS.
In October 2012, Apple announced that the next version of GarageBand, its low-end DAW software, will required that AU plug-ins be able to run "sandboxed", which means that their access to system resources will be restricted. This has created considerable consternation among plug-in developers. First is the problem that sandboxing will require considerable modifications to existing plug-ins to make them compatible, a fear that is exacerbated by the suspicion that Logic, Apple's pro user DAW package, will impose the same restriction in its next release. If other DAW vendors continue to support non-sandboxed AU plug-ins, it could effectively split the AU format into two standards, the sandboxed version for GarageBand and Logic, and the non-sandboxed version for other DAWs that run under OSX, including Reason, Cubase, and Digital Performer. Second, there is a fear that the move by Apple is a prelude to future versions of GarageBand and Logic only accepting plug-ins that have been purchased through the Apple Store, which retains a percentage of all sales. Some plug-in vendors may stop supporting GarageBand and Logic, or abandon the AU format altogether, if this occurs.