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 filesyst

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).