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).