As an addition to my previous post, there is another thing I don't understand.
If I create a simple form with no code behind, and if I chose to publish it to a document library, afterwards, I can use this doc lib to create new list items.
If I take back my form in design mode, I add some code behind to it, I must publish that form as an "administrator approved form" and the final form is available in the "Form Templates" list. As soon as I do that, the link with my original doc lib is broken....So what's the point here?