Hello Pratibha,
the user Exit EXIT_SAPLSLL_LEG_CDPIR3_002 is a functionmodule in ECC where you can modify the data that are send to GTS. The fields I mentioned are fields in the ECC structure /sapsll/api6800_hdr_r3_s and /sapsll/api6800_itm_r3_s. If you fill them they will be automaitcally stored in the corresponding fields of the table /SAPSLL/CUHD and /SAPSLL/CUIT.
In your custom declaration you will find the fields under the tab Administration.
If you want to add new fields you must extend the transferstructure in ECC and the corresponding structures in GTS and, of course do some programming in GTS to fill /SAPSLL/CUHD and CUIT correct.
Regards,
Thomas