Extension folder Configuration/Fluid 

The folder EXT:my_extension/Configuration/Fluid/ contains configuration that introduces namespaces for Fluid.

All files in this directory are automatically included during TYPO3 bootstrap.

New in version 14.1

Configuration/Fluid/Namespaces.php

Namespaces.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Fluid/Namespaces.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Fluid/Namespaces.php

Register and extend global Fluid namespaces.

Read more about Defining global Fluid namespaces.

Configuration/Fluid/ComponentCollections.php

ComponentCollections.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Fluid/ComponentCollections.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Fluid/ComponentCollections.php

This file is used to add Fluid components. It is used to register component collections. For more details see Registering component collections.