Hi Tom, thanks... actually this is exactly what I've done but unfortunately this is not enough.
Ideally I need different fields in billing and shipping address, in this scenario, my client could store correctly billing informations and shipping informations.
Using checkout attributes I won't be able to save data for next purchases.
Having said that, checkout attributes it's my temporary solution but I strongly recommend nop developers to consider the possibility to create different custom fields for shipping and billing address.... for instance: why should i set my email in shipping address?
Nop is amazing but i think that data storage, customization and organization in checkout process could be a bit better... the only weak point I found at the moment.