I installed a clean version of Magento 2.3.4 with composer and added a template. Then I wanted to edit some pages by inserting some XML code into the "Custom Layout XML" field that is visible in this screen:
But in my admin there is shown another field labeled "Custom Layout Update" not XML:
The help tooltip says
List of layout files with an update handle "selectable" matching PageIdentifier_UpdateID. See Magento documentation for more information
But I can't find any information in the documentation and don't know how to enable the "Custom layout xml" field?