Auxiliary Fence

An auxiliary fence is a piece of wood (usually plywood or MDF) attached to a table saw, router table, or other tool's stock metal fence, extending or modifying it for a specific need -- adding height for guiding tall stock on edge, extending length for supporting long workpieces, or simply providing a sacrificial surface a bit or blade can safely cut into without damaging the tool's actual fence.

Because it's cheap, easily replaced, and expected to take direct contact from a cutter in some setups (a router bit buried partway into an auxiliary fence for a stopped cut, for instance), it takes the wear and occasional direct cutting contact that would otherwise damage or misalign the tool's real fence.