If a form is opened as a Dialog form (like a message box) then I don't think you can perform any other operations ntil the form is closed.
Certainly, in code, when you open a form in dialog mode, all code that follows the DoCmd.OpenForm "bla bla" line is not executed until the dialog form is closed which doesn't happen if a form is not dialog (if a form opens normally, all code after the form open code is executed too.