These labels appear in Mirror on even pages context.
They allow the same unique frame style to be used on both left and right pages with left/right relative positioning exchanged.
Suppose you want an icon to be always positioned in the outer margin. This position is traditionally right on right pages and left on left pages. This would require two frame styles but you don’t know whether the frame occurs on left or right page because placement depends on the amount of text preceding the anchor.
In mirrored context, Outside refers to the outer edge of the page, i.e. right on right page and left on left page. Inside is on the inner edge. These words replace Left and Right.
The preview image shows the case of a right page but mentally you must be prepared for the mirror case on facing page.
From Inside measures distances from the inner edge of pages (the binding edge) from left to right on right/odd pages and from right to left (opposite direction) on left/even pages.
This feature is quite handy to balance “landmarks” in outer margins.
EDIT
I filed bug report tdf#159870 and it looks like a duplicate of tdf#65412