AbleCommerce Shopping Cart Software

I have a client that is going to be using Sage Pay. The client has been asked to provide some information about the shopping cart solution that will be used. The question that I need answered is how does able commerce communicate with Sage Pay. The options are by Form, By Server, or Direct.

Can anyone help me with and answer to this question?

Thank you
CP

It communicates via direct API as far as I know.