User exit is commonly used in SD module. There are many exits provided by SAP in the areas of Sales, Shipping, Transportation, or Billing. Generally, user exit is designed to make some changes when standard SAP is not capable to fulfill the requirements. And surely, user exit is contains of some lines of code, so little understanding of ABAP is very needed when you want to write your own exit (especially when there is no ABAPER available..haha..). User exit
To be able to access what exits are exist in each areas of sales, you can go to IMG using this path: IMG > Sales and Distribution > System Modifications > User Exits. Here, any documentations for each exit available in the areas of SD that you want to modified is explained.
For example, I want to show how to find exit available in Sales Document Processing (contract, quotation, or sales order). Just follow the path above and continued to expand the node User Exits in Sales > User Exits in Sales Document Processing.
Click on icon documentation (file & spectacles) to see user exits available in Sales Document Processing.

Read the list carefully, then choose the appropriate user exit you want to apply.

For this example, I used user exit MV45AFZB. Use transaction code SE38 and click Display.

Click Enhance.
Right click on ” ” the choose Create Implementation. If there is sign ” ” appear, right click anywhere in section FORM USEREXIT_CHECK_VBAK then choose Show Implicit Enhancement Options.

Choose Type of Enhancement: Code.
Continued with Create and fill the appropriate field. Then choose Enter.
Fill the appropriate package used. Then save.
Choose the Enhancement Implementation you already created before, then click Enter.
The segment of enhancement in FORM USEREXIT_CHECK_VBAK is created. Write down the line codes below.
good info..very helpful, thanks for posting it in detail.
thanks very clear and detail with screenshots,,,