Is there docs, or API info on what the extend() should take?

What are the default values for the extend()?

function extend($blog_id, $extend, $gateway = false, $level = 1, $amount = false, $expires = false)

I am attempting to programmatically add a site to Pro Sites via another function, it is working but not sure I am getting values correct.

1. Is $extend just a time frame?
2. What is $gateway?
3. Can $level be a variable passed in via $meta array?
4. What is $amount for? Why false by default?
5. What is $expires for? I set it to true thinking that meant trial but no...