Yes you can use RTAS and HTDM on a TDM system. No they are not the same. There are certain limitations to using RTAS Plug-Ins on a PT 5.1 TDM system. In particular:- RTAS Plug-Ins can only be instantiated on disk tracks, not on aux tracks or on master faders.- Even on disk tracks, all RTAS Plug-Ins must be inserted before TDM Plug-Ins.- RTAS Plug-Ins do not support sidechain inputs on 5.1 TDM systems.- RTAS Plug-Ins must have symmetric I/O (i.e. 1 in, 2 out is not allowed)HTDM Plug-Ins do not have any of those limitations.