A physical disk slice is always exported as a single-slice disk. In that case, virtual disk drivers (vds and vdc) forward I/O from the virtual disk and act as a pass-through to the physical disk slice.
A physical disk slice is exported from a service domain by exporting the corresponding slice device. If the device is different from slice 2 then it is automatically exported as a single-slice disk regardless of whether you specify the slice option. If the device is the slice 2 of the disk, you must set the slice option to export only slice 2 as a single-slice disk. Otherwise, the entire disk is exported as full disk.