Checking confval fields 

A field of .. confval:: is matched by its exact name. So :Default: , :default: and :de-fault: are three different fields, and only :default: is the default the directive knows. The others fall through without a word: they are shown as fields of their own, listed in confvals.json under their own name, and empty as a column of a .. confval-menu:: .

A manual can name the fields it uses, and the rendering warns about every other one:

Documentation/guides.xml
<extension class="\T3Docs\Typo3DocsTheme\DependencyInjection\Typo3DocsThemeExtension"
           interlink-shortcode="t3tca"
           confval-fields="TCA path, Scope, Types, RenderType"
/>
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The names are separated by commas and may contain spaces. The check is off until a manual sets them, and a warning fails a render with --minimal-test , which is what the pipelines of the manuals run.

What is checked 

Fields of .. confval::
Every field that is neither declared nor one of those always allowed.
Columns of .. confval-menu::
Every column that names such a field, since it would stay empty.

Always allowed are the fields the directive reads itself, :name: , :type: , :default: , :required: and :noindex: , and those the theme reads for the search, :searchFacet: and :searchKeywords: . The options of a site set's settings are not checked: the theme writes them itself.

A field that differs from an allowed one only in case, spaces, hyphens or underscores gets the spelling that was probably meant:

The confval "cols" has the field "Default", which the manual does not
declare. Did you mean the built-in "default"?
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A rule for all manuals would not help: their fields differ, and each manual knows its own.