Can be World, Object, Tangent. This is the coordinate space where the vector is applied. The default is Object. If so, there are three ways of defining the tangent:
If enabled, inverts (1-channel) the selected input channel.
Input attribute for connecting a tangent vector map.
First, you must set the Subdivision Type of the object to Catclark and increase the Subdivision Iterations (Arnold Parameters > Subdivision). Start off with low values and steadily increase the number until you achieve a good quality displacement effect. Subdividing the base mesh is essential. Otherwise, you will start to see faceting and normal mismatch issues when using low polygon base meshes.
In this case, we have exported a preset vector map from Mudbox and rendered it with Arnold. It was exported as a 32-bit EXR image to retain the highest quality for the vector displacement effect.
Create Arnold Displacement port from the Arnold output port menu (red square).
Final vector displacement shader settings
The image is computed in tangent space, which works well in conjunction with deformed geometry (see the image below).
Further example: Sea waves are displaced using the vector map (right)