A drive-in spike is driven into suitable soil, while a bolt-down support is anchored through a base plate onto a suitable hard surface. Both types must also match the square section of the wooden post.
The main difference is the surface they are designed to fix into.
A 100 x 100 x 600mm Drive-In Fence Spike has a 600mm spike that is driven into suitable ground. The square holder remains above ground and accepts a compatible 100 x 100mm wooden post.
A 100 x 100mm Bolt-Down Post Support does not have a ground spike. Instead, its base plate is anchored to an existing suitable hard surface, with the post held above that surface.
Both fixing types are available for common 75 x 75mm and 100 x 100mm timber posts. The holder size must match the actual post section, so a 75mm support and a 100mm support are not interchangeable.
Drive-in spikes are therefore the ground-fixing option within this range, while bolt-down supports are the surface-mounted option. Compare both formats under fence post spikes and supports before selecting the fixing method.