Slices that are used for soft partitions cannot be used for other purposes.
When you partition a disk and build file systems on the resulting slices, you cannot later extend a slice without modifying or destroying the disk format. With soft partitions, you can extend the soft partitions up to the amount of space on the underlying device without moving or destroying data on other soft partitions.
While it is technically possible to manually place extents of soft partitions at arbitrary locations on disk (as you can see in the output of metastat -p, described in Viewing the Solaris Volume Manager Configuration), you should allow the system to place them automatically.
Although you can build soft partitions on any slice, creating a single slice that occupies the entire disk and then creating soft partitions on that slice is the most efficient way to use soft partitions at the disk level.
Because the maximum size of a soft partition is limited to the size of the slice or logical volume on which it is built, you should build a volume on top of your disk slices, then build soft partitions on top of the volume. This strategy allows you to add components to the volume later, then expand the soft partitions as needed.
For maximum flexibility and high availability, build RAID 1 (mirror) or RAID 5 volumes on disk slices, then create soft partitions on the mirror or RAID 5 volume.