This post is mainly for my Patreon supporters. Some of them are having issues while setting up cryptomatte AOVs. This is how you do it.
Create a material node.
Add an AOVs shader -> cryptomatte.
If you are using the Arnold AOVs supertool, add all the cryptomatte AOVs that you need.
In the arnold global settings add the cryptomatte shader to the AOV shaders.
If you are not using the Arnold AOVs supertool.
Create an arnold output channel define node.
Set it to be crypto_material.
Type RGBA.
Add a render output define node.
Set the channel to crypto_material.
Repeat the same steps to create crypto_object and crypto_asset.