See volumes for an explanation of standard volumes and OpenVDB volumes.
Many volume operations, such as Volume Mix and Volume VOP, only process active voxels in the sparse volume. This can be a problem if you know a certain area in space will evaluate to a non-zero value, but it is inactive in your original volume.
The VDB Activate SDF allows one to expand or contract the region outside of the surface (zeroth level set) of VDB signed distance fields. Expanded regions will have voxels set to the appropriate distance value. This node is faster than VDB Renormalize SDF because it assumes that the signed distance field is already correct.
To see the current active region, you can use the VDB Visualize SOP and set it to Tree Nodes, Disabled; Active Constant Tiles, Wireframe Box; and Active Voxels, Wireframe Box.
The name(s) of the VDB volume primitives in the input to be operate upon. See specifying volumes.
Desired radius in voxel units around the surface (zeroth level set). For proper operation, many nodes expect that half-width is at least 3.
The method for smoothing the SDF after each iteration. Later options in the list are slower but more accurate.