Sy-TABIX : SY-TABIX is a syatem variable which stores the index current processing record of an internal table.

SY-INDX : SY-INDEX is a system variable which acts as a loop iteration counter, it stores loop iteration number.

So when you are looping over an internal table, you use SY-TABIX.

LOOP AT ITAB INTO WA. **SY-TABIX stores index number of internal table record ENDLOOP.

When you use DO ENDDO / WHILE for looping, there is no table involved. So you use SY-INDEX.

DO 10 times. **SY-INDEX stores number of iteration of loop ENDDO.

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