V4.6c onwards no longer supports field exits!!! In any case here's how to activate them:

Activation of the field exits and assignment of the dynpros is carried out using program RSMODPRF. For this purpose, the program must be started without parameters (input fields remain blank).  If required, new field exits can be created using program RSMODPRF (see below for documentation).

Field Exits for Data Elements

Description

Maintenance transaction for creating and activating field exits

As part of the extension concept, SAP allows you to install a field exit for each screen field. Similar to the conversion exit, the system branches at the field exit at DCI (DATA COMMUNICATIONS INPUT) to a function module, if the field is an input field and a function code was triggered. The function module must adhere to the following naming convention:

• Prefix: FIELD_EXIT_

• Infix: <Data element name>

• Suffix: _0 to _9 (optional), _A to _Z

The interface is automatically generated during the creation of the function module, and has 2 parameters:

• Import parameter: INPUT

• Export parameter: OUTPUT

After a function code has been triggered the contents of the field are made available to the FB via the "INPUT" import parameter. You can edit this and, when changed, make it available to the ABAP/4 processor in "OUTPUT". The input field contents are usually passed on: you can simply write OUTPUT = INPUT.

If, e.g., you have activated the field exit for the data element 'PROGRAM', then the fields of all the screens, which refer to the data element 'PROGRAM', branch to the function module FIELD_EXIT_PROGRAM. This includes the field 'Program' of the initial screen SAPMS38M0100 in transaction SE38, for example.
If you do not want the function module to be accessed from all screens, assign particular screens to the data element PROGRAM. By allocating an exit number, it is also possible to branch to other function modules with the corresponding suffix _0 ... _Z.

Example: You have assigned screen SAPMS38M0100 and exit number 5 to data element 'PROGRAM'. The corresponding field on SAPMS38M0100 branches to FIELD_EXIT_PROGRAMM_5.

As soon as you activate an exit, the corresponding screens are invalidated.

Note: Field exits are only taken into account during screen generation if the parameter abap/fieldexit = 'YES' is set in the system profile.

Requirement

Set the profile parameter abap/fieldexit = 'YES' before system start.

Limitations

The following ABAP statements are not allowed in the function modules of field exits:

BREAK-POINT,
CALL SCREEN, CALL DIALOG, CALL TRANSACTION, SUBMIT,
COMMIT WORK, ROLLBACK WORK,
COMMUNICATION RECEIVE,
EXIT FROM STEP-LOOP,
MESSAGE I, MESSAGE W.

Note:

• Field exits cannot be edited with the debugger.

• You should not use any external Performs in field exits. The ABAP processor shares the table work areas of externally called programs with those of the first caller. This could lead to errors at field exits that can hardly be analyzed, as field exits are sometimes slipped into the flow of the program at unforeseen events.

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