EXR Multilayer files can contain any number of additional layers, typically created in CGI packages like Maya and others.

Most Mistika effects only support the traditional RGBA channels (plus Stereo3d).  But the extra layers from EXR files can be accesed in several ways:

- Using the Layer Route effect. which permits to route EXR layers to standard RGBA channels fopr being used in other effects.

- Using the Spatial Keyer effect, which is specialised in selecting parts of the image based on EXR multilayer content. It also provides access to depth information from Stereo3D.

- Using the Color Grade effect, which  has the Spatial Keyer integrated it for easier access.

When using those effects with EXR multilayer files, the Multilayer manipulators will appear automatically in the Visual Editor.

You can find all the details about the parameters of those effects in the Mitika reference manual.  

Note: When using complex EXR files it is recommended to change EXR performance mode in mConfig to Mutllayer mode rather than Single Layer. This will prevent Mistika to read ahead the complete EXR files, thus only reading the neccesary layers where they are used. (In Simple Layer mode  Mistika will decode several  EXR files in advance, which is good for realtime playbacks of simple cases but not for complex multilayer files )

You can find some examples In these videos:


Regarding the EXR file content, here are the general specs of the EXR layer attributes in Blender/Maya as an example. 

Polygon Normals:
This is a standard RGB layer, except that it holds surface normals (vector direction) information in place of R G and B.
In Blender, this is achieved by enabling the "Normals" render pass.

This will be an integer number that represents some given attribute. In the example file, there is an ObjectID layer, which provides a unique mask number for each individual object. There is also a MaterialID, which provides a unique number to represent each texture in the scene.
In blender, this is created by entering a "pass index" in the object or materials property panels. 

This is a standard luminance scale to depict the distance from the camera. 
In blender, this is the "Depth" render pass

This is an RGB layer - except that it contains X,Y and Z positional information instead of colour.
In blender, there didn't seem to be a direct way of adding this as a render pass. So the example file was done by creating a texture where the colour of the relevant pixel equalled the X,Y and Z position of it. 

With some of these layers, don't be surprised if you just see white or black initially. The EXR files can handle very large numbers that exceed the standard luminance scale, so to view them, you will need to bring them into the normal viewable range. You don't need to do this to use the data, just to view it as RGB.