The Corona Cryptomatte is a render element that will help you easily create color mattes of a complex scene with hundreds of objects.
It uses three methods to automatically generate the render channels stored in a multichannel OpenEXR file which removes the hassle of creating dozens or hundreds of single masking elements.
To extract the mattes from still images you need to use specialized software. One example is the free Exr-IO plugin for Photoshop (you can download it here). Without the plugin, Photoshop won't be able to extract the Cryptomatte data correctly.
Currently, to use the Cryptomatte element properly, you must save the element in the .CXR file format from the Corona VFB, rename it to change its extension to .EXR and then load it in your preferred compositing software.
Example of the "Beauty" render image.
What the Cryptomatte pass looks like when rendered.
Note: what you see in the VFB is just a preview. It is not an accurate representation of the data included in the Cryptomatte element. Some visual artifacts may appear (e.g. when using motion blur and DOF), and the colors may not appear to be distributed correctly, but this does not affect the usability of the element in post-production.
How to use the Corona Cryptomatte?
To start using the Corona Cryptomatte, simply follow the below steps:
1. Press F10 in 3ds Max to open Render Setup and then navigate to the “Render Elements” tab: 
2. Click on the add button and select the CMasking_Cryptomatte render element:
In the Render Setup Window > Render Elements > Add > CMasking_Cryptomatte.
The Corona Cryptomatte Parameters:
Levels: Determines how many different objects can be identified by the Cryptomatte render element in a single pixel. By default, this value is set to 6.
ID Type: This parameter determines how many objects are distinguished in the Cryptomatte pass.
1. Name: Using this mode, only objects with the same name will have the same color. This means that if all of your scene objects have different names, different colors will be assigned to each one of them.
Example:
This is the resulting Cryptomatte pass when the ID type is set to "Name"
2. Hierarchy: Using this mode, only objects with the same name and in the same group hierarchy in the object manager will have the same color.
Example:
This is the resulting Cryptomatte pass when the ID type is set to "Name with hierarchy"
3. Layer Name: Using this mode, only objects in the same layer will have the same color.
This is the resulting Cryptomatte pass when the ID type is set to "Layer"
Notes:
- What you see in the VFB is just a preview. It is not an accurate representation of the data included in the Cryptomatte element. Some visual artifacts may appear (e.g. when using motion blur and DOF), and the colors may not appear to be distributed correctly, but this does not affect the usability of the element in post-production.
- Currently, to use the Cryptomatte element properly, you must save the element in the .CXR file format from the Corona VFB, rename it to change its extension to .EXR and then load it in your preferred compositing software.