Extension folder Configuration/TCA 

The folder EXT:my_extension/Configuration/TCA/ can contain or override TCA (Table Configuration Array) data.

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

Files in Configuration/TCA/ are loaded in a dedicated scope. This means that variables defined in the files cannot leak to other TCA files during the TCA compilation process.

Configuration/TCA/<tablename>.php 

Configuration/TCA/tablename.php

tablename.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/TCA/tablename.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/TCA/tablename.php

One file per database table, using the name of the table for the file, plus ".php". Only for new tables, provided by the extension itself.

Must not be used to change existing tables provided by other extensions.

Changed in version 14.0

The backwards compatibility for usage of $GLOBALS['TCA'] in base TCA files is removed. See Important: #107328 - $GLOBALS['TCA'] in base TCA files.

Using $GLOBALS['TCA'] was discouraged before this change and is impossible as the global is not set starting with 14.0. It remains possible to change this global variable in TCA Overrides for now.

Configuration/TCA/Overrides/somefile.php 

Configuration/TCA/Overrides/somefile.php

somefile.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/TCA/Overrides/somefile.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/TCA/Overrides/somefile.php

For extending existing tables.

General advice: One file per database table, using the name of the table for the file, plus .php. For more information, see the chapter Extending the TCA array.