An internal line number in index tables (standard tables and sorted tables). Each table line in an index table has a unique line number, which is managed by the ABAP runtime environment. The line numbers start at 1 and run consecutively and with no gaps, up to the current number of rows in the table. If necessary, the physical index is changed to a logical index, which causes an increased runtime demand. The logical index is created in precisely the moment in which a row is inserted before another row, the sequence of table rows is changed, or if any row except for the bottom row is deleted. A logical index is not required as long as an internal table is exclusively filled with APPEND and if its bottom row(s) are exclusively deleted with DELETE.