Many data entry fields require that you enter a certain value before you can exit that field. For example, you can’t enter a invalid date such as 13/32/91 in a date field. Others require that you enter something, that is, you can enter anything you want, but you may not leave the field blank. Press <F1> on the field to get help on what are valid entries for the field with which you are having a problem, or consult your manual.