I've downloaded the txtr app for my iPod touch, but I'm confused about two things:

How I should fill in that field? I just see blank fields with no guide as to what goes where.
Is it mandatory that I fill it even if I limit myself to purchase books free of charge?

I received an email from them saying that their payment system only works in Germany right now. If you leave it blank, it works fine with non-DRM epubs.