Technical requirement
The function module /GUIXT/Call, the central entry point of the Open Call Interface, is needed in the SAP system. You can import  /GUIXT/Call  via the transport request sap.trans.guixtutilities.zip (click to open up the .zip file). 

Notation of Call statement
The notation is slightly different compared to the normal Call, thereby indicating that you are using the Open Call Interface. Example:

Call "BAPI_SALESORDER_GETDETAILBOS" _
   export.SALESDOCUMENT="aufnr" _
   import.ORDERHEADER="orderheader" _
   import.ORDERITEMS="orderitems"

The Open Call Interface uses GuiXT table variables.

Examples
See the following articles for examples and a detailed explanation of the various possibilities.

Benefits

• Automatic data conversion
  GuiXT works on the UI level where all fields are shown and expected in external format; so it is more appropriate to use this format instead of having to apply the SAP conversion rules. For example, you can pass a customer number as "1284" instead of "0000001284". 
  This also works for more complex conversions, for example when you pick up a  functional location "SWC-EC" from a field on the screen and want to call up the function BAPI_FUNCLOC_GETDETAIL to read some details. With the standard Call you need to pass the internal value "?0100000000000005483", i.e. you first need to call up the appropriate SAP conversion function. With the Open Interface you simply pass "SWC-EC". The same applies in the opposite direction, for example when you use BAPI_FUNCLOC_GETLIST in order to receive a list of functional locations, you will obtain "SWC-EC" in the result table and not "?0100000000000005483" as in the standard call.
  
  The automatic conversion is applied to single fields, fields in structures and to table columns, and is applied in both directions.
  
• Column selection for result tables and structures
  Some BAPIs return tables with a large number of columns. For example, the structure BAPISDITBOS which is used to return a table of customer sales order items consists of 237 columns. In a concrete GuiXT application you may need 24 columns only. The Open Call Interface reduces the network traffic since it transports these 24 columns instead of all 237.
  
• Bundle multiple calls
  You may bundle many calls into one. For example, let us assume that you have a table with 200 sales orders and you want to apply a BAPI for each sales order,  reading additional data. If the RFC network overhead takes 20ms and the internal runtime of the call is 2ms, it will take 4.4 seconds to execute all calls. When you bundle the calls it will be completed in 0.42 seconds.
  
• Parallel processing
  You can switch on parallel processing for the bundled calls. The calls will then be internally  distributed to several SAP work processes. This approach makes sense for long-running calls which can be parallelized.
  For example, when you have 10 calls each needing around 12 seconds, you need 2 minutes with sequential processing versus 24 seconds with parallel processing in 5 work processes (SAP standard configuration). For special cases you can configure more parallel modes in SAP (up to 15) and come down to 12 seconds for all calls. It is even possible to use multiple application servers in parallel, although this is probably rarely needed in GuiXT applications.
   
• Avoid the metadata overhead of the NWRFC interface
  The SAP NWRFC interface, in contrast to the legacy "classic" RFC, needs meta data which describe the function module interface. Reading the metadata is done the first time a function module is called after starting SAP GUI; this can take several seconds for each function module. The Open Call Interface avoids this overhead, since the functions are called internally by /GUIXT/CALL.
  
• Avoid problems with non-ANSI characters in small fields
  If you want to set e.g. "ÖX" into a 2 character field of a BAPI structure, only "Ö" will be transmitted in the standard call, since GuiXT uses the SAP-UTF-8 structures which reserve one byte only for each character. This problem no longer occurs with the Open Call Interface.
  
• Call up function modules which are not remote-enabled
  Since the Open Call Interface calls up the function modules internally, not via RFC, it is not required that they are marked as "remote-enabled" in transaction SE37. For security reasons the Open Call Interface nevertheless checks the user rights for calling the function module remotely so that you can control which function modules should be callable by GuiXT (authorization object S_RFC).
  
  There exist many SAP function modules which are useful in GuXT applications but are not released for RFC. With the Open Call Interface it is no longer necessary to create "wrapper" functions in order to call such function modules from GuiXT, since /GUIXT/Call plays the role of a generic wrapper function.
  
  There is also a "Released" attribute in SE37 which is independent of "RFC enabled". "Released" means "Released for customer's use". SAP discourages you from using function modules that are not "Released", since such functions can be changed without notice and without SAP taking over any responsibility or providing OSS support, unless the problem occurs in a standard application. For details see OSS note 109533.
  
  On the other hand, even well-documented and popular function modules such as MATERIAL_UNIT_CONVERSION are not "released". Some statistics: There exist around 500 000 function modules, 8 000 of them are released. Of 48 000 RFC enabled functions 4 000 are released.  There are around 5 000 BAPIs and only 3000 are released, which looks like a 'contradictio in se'. On the other hand, there exist 4 000 function modules which are "Released" but not "RFC enabled".
  
  For example, the function modules READ_TEXT and SAVE_TEXT  are "Released"  and well-documented. But they are not RFC enabled. There exist the counterparts RFC_READ_TEXT and RFC_SAVE_TEXT which are RFC enabled, but  they have a different interface, no documentation in SE37 and are not "Released". The Open Call Interface gives you direct access to the functions READ_TEXT and SAVE_TEXT.
  

Components
InputAssistant