What system variable contains the implicit loop counter when processing a loop with DO... ENDDO?

In the context of ABAP programming, when you are using a loop structure such as DO... ENDDO, the system variable that acts as an implicit loop counter is sy-index. This variable automatically increments with each iteration of the loop, which allows you to track the current iteration number without needing to manually manage a separate counter.

The use of sy-index provides a convenient way to reference the iteration index directly in your loop without additional coding. This is especially useful when you need to access array elements or perform operations conditional on the current iteration.

Other system variables like sy-linno, sy-tabix, and sy-subrc serve different purposes. For instance, sy-linno typically relates to line numbers in a list display, sy-tabix deals with the index of the current row in an internal table, and sy-subrc is used to check the return code of certain operations. These variables do not function as a loop counter in the context of the DO... ENDDO structure, making sy-index the correct choice for this question.