USB devices can definitely be formatted with APFS, since they did that on-stage during the demo. I'm less sure about iSCSI targets, but iSCSI is a block-storag

USB devices can definitely be formatted with APFS, since they did that on-stage during the demo. I'm less sure about iSCSI targets, but iSCSI is a block-storage protocol. If you can format an iSCSI volume with HFS+ today, you'll probably be able to format it with APFS tomorrow.