What variables are available to redirectToPostedUrl?
When you take a deeper look into redirectToPostedUrl you'll see these lines. All they do is convert the Object into an array, so they fetch every property of the object and pass those to the template to use them directly. You have everything the Object has.

You can access those attributes via short syntax {title} which is the same as {{ object.title }}. 

For example if you have an yii/model/MyFancyModel with the following attributes

[
    'id' =&gt; 4,
    'title' =&gt; 'foo',
    'someRelation =&gt; ['foo' =&gt; 'bar']
]


Craft will merge this array with ['object' =&gt; $object] and you have access to { id }, { title }, { someRelation.foo } as well as {{ object.id }} (and all others) as well as all other variables passed via variables the third attribute of the function, since redirectToPostedUrl doesn't pass a third argument these additional attributes are an empty array.

In your case { siteUrl } is nothing else than {{ object.siteUrl }} which is basically {{ entry.siteUrl }} that means the siteUrl of your Element. When you do { type.handle } (with an Entry)  it will render the handle of your EntryType. When your Object is a user you can do { email } to display his/her email.

One more note: The attributes of the Model are fetched via Arrayable Trait if it has it. So it grabs all fields and extraFields gets those attributes. I can provide further explanations about fields and how they work exactly in detail if you like