Arnold uses something called LPE(Light path expression) to set AOVs,Using those expressions we can seperate the AOVs per light group basis.

• • set some name AOV Light group parameter in the arnold light

  • create AOV in arnold render node

  • set the LPE to get desired results.

  • Example:

    • beauty per lightgroup - C.*<L.'lightgroupname'>

Here are the available expressions - https://support.solidangle.com/display/A5ARP/Light+Path+Expression+AOVs

To export custom attribute as crytomatte, need to make sure few things as follows

• Attribute needs to be Primitive attribute, point attributes simply doesnt work.

• Attribute needs to  be a string attribute

  • for example :  you cannot use @id  to create custom cryptomattes,  instead you will need to create  new string attribute  something like s@piece_id  =  "piece_1"