They're used for cutting borders (5)
I believe the answer is:
edges
I believe this is a double definition.
'they're used for cutting' is the first definition.
(I've seen this before)
'borders' is the second definition.
(an edge is a border or margin)
(Other definitions for edges that I've seen before include "Bounds" , "Boundaries of a surface" , "Outer limits" , "Margins" , "Outside limits" .)