RTAS plug-ins use the processing power of the host computer rather than DSP cards used in the Pro Tools HD systems.
[1][failed verification] As the name suggests, the plug-in architecture is designed to be run in real-time, mimicking hardware inserts on traditional mixing console.
This is in contrast to rendering files out of time with effects applied directly to the audio, which in Pro Tools is facilitated by AudioSuite Plug-ins.
[2] Avid's AAX format, which runs on both native CPU and Pro Tools HDX DSP, is the replacement for RTAS.
From this version forward, Avid has made the AAX plug-in the only format supported by Pro Tools.