from a quick look at the code: https://git.proxmox.com/?p=pve-container.git;a=blob;f=src/PVE/LXC/Setup/Base.pm;h=dbfc775c6f4769609c50caa6c8a7b91a91566080;hb=HEAD#l294
It should be set to true, if you set the containers IPv6 config to 'auto' (called SLAAC) in the GUI (did not test this explicitly...