By enabling the Object ID Output option the Object ID texture map allow to provide to the Action Output Object ID render pass the Surface Object ID information, so you are no longer limited to having only a single object ID for the whole Surface.
Blending between ids has to be defined and bit depth mismatch has to be handled (input 32bits map -> 16bits action output). New 'Preserve 32-bit Input' option in Object ID render pass UI menu.
To add an Object ID map:
In the schematic, select the surface or 3D geometry to which you want to apply the Object ID.
Click Media.
In the Media menu, select the media you want to use for the Object ID.
Do one of the following:
The Object ID object is added to the schematic with its own parent axis. The new axis is the child of the selected surface or geometry. In Schematic view, the number in brackets next to the name of the Parallax node indicates the media used for the Object ID.
To specify different media as the source, select the media in the Media menu, then click Apply, or use the Read File tab to manage the texture media (see Working With Textures in Map Nodes).
Double-click the Parallax node in the schematic, or follow the tab population rules for the Object menu (see Populating Menu Tabs of Selected Objects).
The Parallax menu appears.