When you create a UFS snapshot, you can specify that the backing-store file is unlinked, which means that the backing-store file is removed after the snapshot is deleted. If you don't specify the -o unlink option when you create a UFS snapshot, you will have to delete it manually.
The backing-store file occupies disk space until the snapshot is deleted, whether you use the -o unlink option to remove the backing-store file or you remove it manually.