Introduction to Pro Tools : Pro Tools Concepts : DAE : Playback Engine

Playback Engine
Pro Tools lets you adjust the performance of your system by changing system settings that affect its capacity for processing, playback, and recording. These system settings are available in the Playback Engine (Setup > Playback Engine).
Pro Tools takes advantage of your computer’s host processor for certain tasks and optional host-based plug-in processing.
Pro Tools uses host (CPU) processing to provide audio track recording, playback, mixing, and effects processing. Both Pro Tools and Pro Tools HD use host-based processing to run Native AAX (Avid Audio Extension) and RTAS® (Real-Time AudioSuite) plug-ins for effects processing. Performance is determined by your system and its Playback Engine settings.
The Playback Engine lets you set a hardware buffer size and allocate a percentage of CPU resources for these tasks.
 
Playback Engine for Pro Tools|HD system
On Pro Tools|HD systems, you can select the number of voices and voiceable tracks for your system and its sessions. Voice count choices are based on how much DSP processing you want to allocate for voicing.
The Playback Engine is also where you assign dedicated resources for Automatic Delay Compensation.