1.      Enhancement is a concept by which we can add our custom code to standard SAP software without modifying it .
2.      Each enhancement point =  Exit point.

Customer Exit are  ZINCLUDE Programs which are available inside a Function Module.
 Fuction Module  = Customer Exit
So We Can Call Customer Exits as Enhancements ,why because it does not require any access key.
We can get Customer Exit in all Modules,where User Exits are available in SD module only.
we have 4 types of Customer Exits
1. Function Module Exit
2. Menu Exit.
3. Screen Exit
4. Field exit

We can search customer exit in 3 ways.
1. SE93 : Go to Tcode SE93 and Provide Tcode XK01.

Then Click On Display Button.

Now just Double Click On Program Name  and Click On Find Icon and provide Name As CALL CUSTOMER In Find Option And Select As String Radio Button And Select In Main Program Radio Button And Select Enter   .

 Continue Till All Call Customer Exits Displayed


Just Double Click On Call Customer We will get Function Exit and at the Bottom We can Find our INCLUDES  Dobule Click on it and  write our custom code

2. SMOD:  Finding Customer Exit using SMOD
1. At first we need to know the package of a particular Tcode, So Go To Tcode SE93  

Just Copy The Package Name 

3. Now go to Tcode SMOD 
4. Click on Find Icon

5.  And Provide the Package Name , Click ON Execute Icon

6. We will get List of Customer exits.

3. Using SE84 :
 1. first need to go Tcode se84.
 2. Expand the Enhancement folder.
3. Expand the Customer Exit folder.
4. Need to double click on Enhancement folder
5. Provide the Package Name of a particular transaction, finding Package for Tcode is Done By using SE93 .
6. After providing Package Name Just click on Execute button.

 we will get list of exits