If the slice option is set, then the file or volume is exported as a single slice disk. In that case, the virtual disk has only one partition (s0), which is directly mapped to the file or volume backend. The file or volume only contains data written to the virtual disk with no extra data like partitioning information or disk structure.
When a file or volume is exported as a single slice disk, the system simulates a fake disk partitioning which makes that file or volume appear as a disk slice. Because the disk partitioning is simulated, you do not create partitioning for that disk.