>>>>>>>>>>
>What's the behaivor diferences from a modal form
> to a non-modal ?
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To begin with, a modal form stays on top and you cannot click or select other windows until that one is closed. It goes against the grain of the Windows philosophy where you should allow you users to be able to switch windows without closing them first, and even, if you want, you can allow them to open one window more than once (e.g. look at two customer records side by side).

That said, modal windows have their place, like MessageBox with errors that you have to acknowledge and child forems for data-entry. For example, a form with a read-only grid for data navigation, and when you select a record, it opens a modal window with controls to edit that one record. You must save and close before being allowed back to the navigation grid.