I think that feature was in older versions but I don’t see it now.
So basically that option will come with that “cursor” feature in item lore.
It will insert an new lore line under location of “< Selected” cursor.
So when you have lot of lines and you want just to insert in the middle of the text, you don’t need to remove all lores under that line you want to add, you just use “Insert Line” option. I swear it was in older versions of UltraCustomizer 😂