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.