CMOD is the Project Management of SAP Enhancements (i.e., SMOD Enhancements). SMOD contains the actual enhancements and CMOD is the grouping of those SMOD enhancements. 

User exits (Function module exits) are exits developed by SAP. The exit is implementerd as a call to a function module. The code for the function module is written by the developer. You are not writing the code directly in the function module, but in the include that is implemented in the function module. 

The naming standard of function modules for function module exits is: 
EXIT_<program name><3 digit suffix> 

The call to a functionmodule exit is implemented as: 
CALL CUSTOMER.-FUNCTION <3 digit suffix> 

For Example: 

The program for transaction VA01 Create salesorder is SAPMV45A

1. If you search for CALL CUSTOMER-FUNCTION program SAPMV45A you will find ( Among other user exits): 

CALL CUSTOMER-FUNCTION '003'
  exporting
    xvbak   = vbak
    xvbuk   = vbuk
    xkomk   = tkomk
  importing
    lvf_subrc = lvf_subrc
  tables
    xvbfa = xvbfa
    xvbap = xvbap
    xvbup = xvbup. 

The exit calls function module EXIT_SAPMV45A_003 

2. How to find user exits 

Display the program where you are searching for and exit and search for CALL CUSTOMER-EXIT 

If you know the Exit name, go to transaction CMOD. Choose menu Utillities->SAP Enhancements. 

Enter the exit name and press enter. 

You will now come to a screen that shows the function module exits for the exit. 

or use this ABAP program to search for user exits :-

Finding the user-exits of a SAP transaction code

3. Using Project management of SAP Enhancements 

You want to create a project to enhance transaction VA01

- Go to transaction CMOD
- Create a project called ZVA01
- Choose the Enhancement assign radio button and press the Change button
In the first column enter V45A0002 Predefine sold-to party in sales document . Note that an enhancement can only be used for 1 project. If the enhancement is allready in use, and error message will be displayed
- Press Save
- Press Components. You can now see that enhancement uses user exit EXIT_SAPMV45A_002. 
- Double Click on the exit.
Now the function module is displayed. Double click on include ZXVVAU04 in the function module
Insert the following code into the include: E_KUNNR = '2155'. 

Activate the include program. Go back to CMOD and activate the project. 
Goto transaction VA01 and create a salesorder. Note that Sold-to-party now automatically is "2155" 

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