It means the gravel board is the separate timber component positioned closest to the ground, taking the moisture, soil contact and splash exposure that would otherwise affect the main fence panel.
Calling a wooden gravel board sacrificial does not mean it is intended to fail quickly. It describes its position as a separate, replaceable timber component between the main fencing and the ground.
Our 1.83m wooden gravel board is 150mm high and 22mm thick. It sits beneath a compatible fence panel so the gravel board, rather than the bottom of the panel, is closest to soil, damp vegetation and rain splashback.
The practical advantage is separation. If the lowest timber component eventually needs attention, the gravel board can be dealt with independently rather than making the bottom edge of the main fence panel the component directly exposed at ground level.
Our wooden gravel boards are pressure treated and can be cut to suit suitable timber fencing layouts. Freshly cut ends should be re-treated before installation.
This is also why a gravel board is a separate part of the fence height. The 150mm timber base sits below the panel and contributes its own height to the completed boundary.