The main purpose of SMOD is to display the enhancements to a particular package in enhancement we have function exits, field exits and screen exits. it is only display purpose
go for smod transaction in enhancement field press f4 function in serch give the package name of paricular t-code suppose va01 package is VA. Then you will get no of enhancements be reading desciption you will come to know what is the suitable enhancement.
In CMOD create a project and give the enhancement what ever you found the enhancement in SMOD you will get no of exits go into the particular function modules exit, that function module source code you can find include program starting with 'ZX' in that program you will write code and activate it.
Normally these is a work of Technical Person (ABAPer).
Purpose of ENHANCEMENTS /SMOD & CMOD
Refer to SAP Documentation
https://www.sdn.sap.com/irj/sdn/nw-development?rid=/webcontent/uuid/2342e1f3-0b01-0010-a186-fdd40488... [original link is broken]
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