difference between open_form and start_form

refer to :http://forums.sdn.sap.com/thread.jspa?threadID=824322

Any driver program must contain Open_Form, Close_Form and Write_Form. Other function modules are optional.
OPEN_FORM: This function module opens layout set printing. One must call this function module before he uses any of other layout set function modules like Write_Form, Start_Form, Control_Form etc., You need specify a layout set name in the export parameters of this function module. If nothing is passed to Open_Form then one should use Start_Form function module to open layout set before starting the output. The layout set opened by Open_Form should be closed by Close_Form function module, other wise output doesn’t appear. We can use any number of Open_Form, Close_Form functions in one single program. But Close_Form must be there for every Open_Form in the program.

Some of the important export parameters we have to pass to Open_Form function module.

Form: Enter the Layout set name here, which then controls the output formatting. If nothing is specified then we must use Start_Form for opening layout set.
Language: Layout sets are Language dependent. Enter the desired language here. If the entered layout set is not in that language, then the function tries to open the Form entered in its original language. The default value is SY-LANGU.
Device: SAP Script can format a text on different device types. The device can be ‘PRINTER’ or ‘TELEX’, ‘TELEFAX’, ’ABAP’ and ‘SCREEN’.
Dialog: Use parameter DIALOG to determine whether to display a dialog box before printing, in which the user can set several spool parameters for print formatting. Possible values: SPACE displays no print parameter screen. 'X' display print parameter screen. Default value: 'X'
Options: Use parameter OPTIONS to set several options for print formatting. The parameter has the structure ITCPO. The user can change some of the defined settings on the print control screen. The default value for Options is ‘ ‘. We will discuss about ITCPO structure in detail later.

Other Export, Import and Exceptions are self-explanatory.

Start_Form: In-between the function modules OPEN_FORM and CLOSE_FORM, we can use different layout sets. This allows us to combine several different layout sets into one print output. However, we can combine only those layout sets that have the same page format. To switch layout sets, use the function module START_FORM. If another layout set is still open, we must close it first using END_FORM. If we specify no layout set name when calling START_FORM, the system restarts the last open layout set. If after OPEN_FORM no layout set was activated yet, the system leaves the function module with the exception UNUSED. Some of the important export parameters for this function module are

Form: The parameter contains the name of the layout set you want to use for printing. If you specify no layout set here, the system restarts the last active layout set.

Language: Layout sets are language-dependent. Enter the desired language here. If the layout set does not exist in this language, the system tries to call the layout set in its original language. If the parameter LANGUAGE is empty, the system uses the language of the last active layout set.

Startpage: Usually, SAP script starts with the page specified as start page in the layout set definition. If we want to start output with another layout set page, enter the name of the desired layout set page here. If the desired page is not defined, the system uses the start page defined in the layout set.

Program: To replace program symbols, SAP script must know which active program contains the work areas for the values to be passed. If we omit the parameter, the system searches for the field values in the program that was specified in the parameter OPTIONS (field TDPROGRAM) of OPEN_FORM. If we enter a program name, the system replaces the program symbols with the values from this program up to the next END_FORM.