Sorry, not much help from WHMCS- Looking for info on actionhooks

Here's a copy of the actionhooks.php file from my copy of WHMCS:

<?php
/*
********************** Action Hooks **********************
* Action hooks allow you to write your own PHP code that *
* will run when specific actions occur in WHMCS. There   *
* is a function for each action that supports hooks. The *
* variables avaible in a specific function are listed in *
* the comments for that function. Invalid code here will *
* will cause WHMCS to fail so ensure your code is valid. *
**********************************************************
*/

define("ACTIONHOOKSINC",true);

function actionhook_ClientSignup($vars) {
	# This function runs when a new user signs up through the shopping cart or registration page
	# $vars["ID"]
	# $vars["FirstName"]
	# $vars["LastName"]
	# $vars["CompanyName"]
	# $vars["Email"]
	# $vars["Address1"]
	# $vars["Address2"]
	# $vars["City"]
	# $vars["State"]
	# $vars["Postcode"]
	# $vars["Country"]
	# $vars["PhoneNumber"]
	# $vars["Password"]
}

function actionhook_ShoppingCartCheckout($vars) {
	# This function runs when the user checks out using the shopping cart
	# $vars["OrderID"] - Order ID Number
	# $vars["OrderNumber"] - 10 Digit Order Number
	# $vars["InvoiceID"] - Invoice ID Number
	# $vars["Products"] - Array of Product IDs from Order
	# $vars["Addons"] - Array of Addon IDs from Order
	# $vars["Domains"] - Array of Domain IDs from Order
}

function actionhook_InvoicePaid($vars) {
	# This function runs when an invoice is fully paid and therefore the services renewed
	# $vars["InvoiceID"]
}

?>

Code (markup):

What do you do with it? How do you customize it? I'm looking to automatically make each new registered user's status as "Approved" instead of the default "Pending"

I was told I can do this by using the actionhooks, but there is really no documentation on how to use this.

Any assistance??