Hi,
If you keep your cursor in line 40 and then click on "insert" button, system will propose line number 39. If you click insert again, line 38 will be displayed and so on. If you keep the cursor on line 20 and click on insert, then line 19 will be proposed and so on.
The configuration in ML91 & ML98 controls the initial line interval only. If you click on insert new line, system will propose like 19, 29, 39 etc, based on your cursor position. Its standard system behavior. The insert functionality is to insert new line in between, for a service order which is already created.
Hope you understand the logic.
Regards,
AKPT