Ideally I would like to take a value entered from an Option List and use it in a linked program before the form is submitted,
The value would be used in a select search so that the operator then knows what to format of data to enter in the next field on the un-submitted form. Can unsubmitted form data be used in $_SESSION variables to do this? or is it possible some other way?
PHP normally only sees the data when you submit the form. If you want a PHP program to process the data 'live' then you will need some Ajax to do that. It can get data entered into the fields and call a PHP program to deal with it.