WP Plugin Pluggable Functions

Pluggable functions were introduced in WordPress 1.5.1 These functions let you override certain core functions via plugins. The most up-to-date list of core functions that WordPress allows plugins to override is available at wp-includes/pluggable.php. WordPress loads the built-in functions only if they are undefined after all plugins have been loaded. Pluggable functions are no longer being added to WordPress core. All new functions instead use filters on their output to allow for similar overriding of their functionality. Note: A function can only be reassigned this way once, so you can’t install two plugins that plug the same function for different reasons. For safety, it is best to always wrap your functions with if ( !function_exists() ), otherwise you will produce fatal errors on plugin activation.

find  the pluggable.php inside the folder wp-includes ( wp-includes\pluggable.php) in WordPress DIR.

https://codex.wordpress.org/Pluggable_Functions