System Configuration : I/O Setup : Session Interchange : Path Names and Automatic Output Bus Remapping

Path Names and Automatic Output Bus Remapping
A system ID is created and saved in a session for every computer the session is opened on (using the computer’s MAC address). If Pro Tools finds a matching system ID when opening a session, output paths are restored and no reconfiguration is necessary.
When a Pro Tools session is opened on a system for the first time, Pro Tools attempts to automatically remap output busses.
If Pro Tools does not find a matching system ID, Pro Tools attempts to remap output busses according to the following criteria (in order):
Path Name and Format
Path names must be exactly the same, and of the same format. Use the suggested path naming schemes (see Recommended Output Path Naming Schemes).
Path Format Only
If matching path names are not found, Pro Tools remaps paths to existing paths of the same format (channel width).
Any output bus path that cannot be automatically remapped is opened as Inactive. You must manually remap those paths to an active output path.