Hi Raghav,
how I found the information:
FBL1N , set up a vendor_line_item_display-list
system -> status -> double-click on GUI status ALV_ITEMS_AR to see which characters are used for edit-option = CHNG.
system -> status -> double-click on Program (GUI) SAPLFI_ITEMS and search for (binoculars buttom! at the top, not Ctrl+F) 'CHNG'. The first place shows the dependence of the switch x_change.
Started FB1LN again in debug mode and created a watchpoint for x_change.
Much easier: SAP notes, search for fbl1N and authorization.