How to find the code for the SAP EXIT Variables?

1) Go to SE37 (Function Builder : initial screen)
2) Function Module : Enter the FM name for exp: RSVAREXIT_0P_KEYD2

FM name for the SAP EXIT variables is RSVAREXIT_variablename

Click on Display button. And check the Code in the next screen.

Note:
This FM will be available only for variables having "SAP Exits" as processing type Not for all SAP delivery variables.

Brief Information about the SAP EXITS

You can use transaction SE16 to display all delivered SAP Exit variables in table RSZGLOBV with the
settings OBJVERS=D, IOBJNM = and VPROCTP =4 for a characteristic. SAP Exit variables are predominately used for the variable type characteristic value variable (VARTYP =1). The ABAP coding belonging to a SAP Exit variable can be found in the function module RSVAREXIT_ (se37). Nevertheless, the usual SAP Exit variables for time characteristics are filled using the BW function module RREX_VARIABLE_EXIT (older version). With each new SAP Exit variable, a function module must be created with the name RSVAREXIT_. A user can view/copy the interface in the existing module
RSVAREXIT_OP_FVAEX. The module is to be created in its own function group for the application (such as BWCO for SAPExists in the Controlling area), so that any errors do not influence other programs. For the interface: I_VNAM contains the variable name (redundant as already in the name of the module), I_VARTYP, I_IOBJNM and I_S_COB_PRO give information about the variable and the corresponding InfoObject, I_S_RKB1D and I_S_RKB1F contain information about the query (such as fiscal year variant in I_S_RKB1F-PERIV if not a variable) and I_THX_VAR contains the already filled values of the variables. Here you can find where appropriate values of a variable for 0FISCVARNT providing that I_S_RKB1FPERIV is empty. In table E_T_RANGE, only fields SIGN, OPT, LOW and HIGH are
allowed to be filled. SIGN and OPT are also to be filled for parameter or interval variables (with I and EQ or I and BT).
The variable processing type “Customer Exit” can be used in a similar way like the SAP Exit variables delivered by the SAP Business Content.