open-form: to open a form

start_form:to trigger a form from another form

Open form : to open a form with default page sequence from 1
to x.
start form : its used when u want to take control of the
page sequence to be triggered like u want the 3rd page first
then 5th then 4th if u want such a seq u go with start form.

OPEN_FORM

The function module OPEN_FORM opens form printing. You must 
call this function module before you can use any other form 
function (WRITE_FORM, START_FORM, CONTROL_FORM...).

You need not specify a form name. If you omit the name, you 
must use the function module START_FORM to open a form 
before starting the output.

You must end form printing by using the function module 
CLOSE_FORM. Otherwise, the system does not print or display 
anything.

Within a program, you can use several OPEN_FORM.. 
CLOSE_FORM pairs. This allows you to write output to 
several different spool requests from within one program.

START_FORM

In-between the function modules OPEN_FORM and CLOSE_FORM, 
you can use different forms. This allows you to combine 
several different forms into one print output. However, you 
can combine only those forms that have the same page format.

To switch forms, use the function module START_FORM. If 
another form is still open, you must close it first using 
END_FORM.

If you specify no form name when calling START_FORM, the 
system restarts the last open form. If after OPEN_FORM no 
form was activated yet, the system leaves the function 
module with the exception UNUSED.