The From Sheets option requires that you first create sheet block(s) which will then be used to split the free block.
The sheet blocks used to split the free block are internal faces which pass through the volume. The split sheets can be either contiguous or separated by other faces. However, the selected split sheets combined with other faces of the free block should split the volume into manifold regions. Only those sheets which form the closed volume need to be specified.
Note:
Currently, the sheet faces must be structured. However, other faces of the free block to be split can be of any type (structured or free). A structured sheet can be created from 4 vertices or points (see the Sheet option in the 3D Blocks section).
Free blocks with multiple shells can be split only if the split sheets connect inner and outer shell faces. This will result in single shell manifold volumes after the split.
Split sheets must be connected to the boundary faces, a rectangular region fully embedded inside the free block is not supported.