Cant find anything in the manual or online on how to config this.
The screen is under the advanced tab on the ipv6 DHCP server. (lAN-General setup)
My issue is that I get connected find and get ipv6 from Sky my ISP

However anything south of the 2862N is behind a ubiquti gateway that works fine with ipv4 but cant get an ipv6 lease from my draytek,
It also seems to want to run its own ipv6 dhcp server. It thinks its an edge device, and tries to run its server using the range the draytek is aleady serving.

So I am wondering if I can sub delegate a range here to possibly solce my issue

This is the screen

DHCPv6 Server
Authentication Protocol
Prefix Delegation Enable Disable
Prefix 2A02:C7F:96AB:xxxx::/56

DHCPv6 Prefix Delegation

New Prefix
:
:
:
::/64
Suffix
:
:
:
New Prefix Length
(0~64)
Client Link Local Address
Client DUID(option)

This is my reason for asking.

This quote on the ubiquiti usg congig page.

NOTES & REQUIREMENTS:
DHCPv6-PD will not work with the USG behind another router that does not provide prefix delegation or sub-delegation.
Will not work with 6RD, DS-Lite, or other non-native IPv6 technologies.
Find the manual configuration method used for UniFi Controller software version older than v5.7 at the end of this article. As always, we recommend staying with the newest software and firmware versions at all times.

Wanting to know if the 2862 provide PD or sub delegation.

On this screen enable prefix delegation, put something in the suffix fields (eg. 10 in each of the three boxes), 64 as the new prefix len and the IPv6 link local address of the USG's link (which is connected to the 2862N) in Client link local address and click add. Repeat with eg. 20 in each suffix box and then with 30 and so on. You need to do this the same number of times as network interfaces the USG has (not counting the interface connected to the 2862N). Each one you add will be shown underneath with a delete button in case you made a mistake.

However only the first one you enter will work - see my reply to your other post.