Real Time AudioSuite. Based on Digidesign’s AudioSuite plugin architecture that uses off-line processing, RTAS acts exactly like a TDM plugin , but instead of using Digidesign DSP, uses the host computers processing capability. The upside is that RTAS does not require the expensive hardware that TDM does, but the downside is that the number of instantiations will vary with processor power, and there are certain limitations as to where a plugin can be placed.
(See TDM)