The disklabel utility can be used to install, examine, or modify the
label on a disk drive or pack. The disk label contains information about
disk characteristics (size, type, etc.) and the partition layout, stored
on the disk itself.
disklabel supports 15 configurable partitions, `a' through `p', excluding
`c'. The `c' partition describes the entire physical disk, is
automatically created by the kernel, and cannot be modified or deleted by
disklabel. By convention, the `a' partition of the boot disk is the root
partition, and the `b' partition of the boot disk is the swap partition,
but all other letters can be used in any order for any other partitions
as desired.
The word "slice" does not come up in disklabel's man page at all.
Your openbsd sd0e partition wont show up under fdisk.
sd0c will as it can be maped to either the openbsd A6 partition under MBR/GPT or a disklabel using the whole disk.
As far as I know the c partition is always the whole disk, regardless of what you have (or don't have) in MBR or GPT. If you want to image the whole disk with dd, you'll reach for rsd0c, always.
But I still don't understand what distinction you're trying to make. Yes, disklabel partitions don't show up in MRB, just as GPT partitions won't. No surprise that the three different partitioning methods aren't exactly aware of each other.
For all practical intents and purposes, BSD's use partitions. They just call them differently because they use a different technical implementation. But they're still just as static and just as limiting as regular partitions.
The important distinction for me is that the tools for working with partitions exist on openBSD (eg fdisk) but those deal with mbr/ gpt partitions, and won’t tell you about slices. So practically it’s a distinction you need to understand if you intend on working with them.
It's the same...GPT is the partition-table, you even write it yourself:
>into several parts
parts are partitions and not subvolumes, a subvolume is a filesystem or volume-manager on top of a partition and it's size/quota etc is defined BY the filesystem or the volume-manager (look up btrfs subvolumes, zfs-filesystems or lvm).
http://man.openbsd.org/disklabel