Script Extension: When does the 'extension loading into memory' event occurs?

As a test, I created a script unit for a script extension with just the following:

var mySomething;

function getMySomething() {
    return mySomething;
}

function setMySomething(value) {
    mySomething = value;
}

function Initialize() {
    mySomething = 'this is the initial value';    
}

I encapsulated this into an extension with the following Description.XML

<ScriptExtensionGroup Name="TestThis">
    <Category Name="TestThat">
        <ScriptExtension Name="TestInitialization" Author="Robert Martin" Version="1.0">
            <Script Name="testExtensionInit.sj" InitRoutine="Initialize">
                <RuntimeObject Name="TestInit">
                    <Property Name="MySomething" GetRoutine="getMySomething" SetRoutine="setMySomething">Property description</Property>
                    <Description>Runtime object description</Description>
                </RuntimeObject>
            </Script>
            <Description>Script extension description</Description>
        </ScriptExtension>
    </Category>
</ScriptExtensionGroup>

I then wrote this code:

function blah() {
    Log.Message(TestInit.MySomething);
    TestInit.MySomething = 'New Value';
    Log.Message(TestInit.MySomething);
}

The first time I run this code after starting TestComplete I get a log that looks like this:

If I run it again without restarting TestComplete, I get the following:

So...  the Initialization is executed at loading of TestComplete. The value in the variable persists beyond the test run so long as TestComplete has not been restarted.

If you want a variable value to be initialized in a script extension at the start of a test run, I believe you'll still need to call specific code to initialize the value.