IPv6 address labels are used for address selection; they are described in RFC 3484.
Precedence is managed by userspace, and only the label itself is stored in the kernel.
enp0s25.network
[Match]
Name=enp0s25
[Network]
DHCP=yes
Address = 2001:db8:f00:baa::b
[AddressLabel]
Label=199
Prefix=2001:db8:41::/64
[AddressLabel]
Label=11
Prefix=2001:db8:31::/64
[AddressLabel]
Label=123
Prefix=2001:db8:21::/64
[AddressLabel]
Label=124
Prefix=2001:db8:11::/64
[sus@maximus label]$ ip addrlabel list