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Render Settings window
The full functionality of management of Render Passes is explained in details in Managing Render Passes. Here is a description of the two items in the Render Settings window for selecting and creating a Render Pass:
Current Render Pass
This option menu lists all Render Pass Passes currently in the scene. Select one to make it active and display all its attributes right below. The selected one is also used for all renderings launched using Render Current Frame or Batch Render options of Maya’s Render menu. By default there is only one Render Pass and it is named 'renderPass1renderPass'.
Create Render Pass
The Create Render Pass button creates a new Render Pass and sets it as the Current Render Pass.