Hello Brian
MAC addresses when configured have the U/L bit which is the 7th bit of the 48 bit address. This bit, when set to 0 when this address is locally administered and 1 if the address is globally unique. An example is the virtual MAC address that is created by HSRP. This MAC address will always have 0 in the seventh bit, while a hardwired MAC address on a switch or a PC will have the 7th bit 1.
Now because there is a mechanism of EUI-64 which is used to assign an IPv6 address that is derived from a MAC addresses, this L bit seems to have migrated into the IPv6 format. According to most sources, (and @ReneMolenaar can correct me if I’m wrong), this bit isn’t actually taken into account in most IPv6 applications. Both the FC00 and FD00 blocks are considered unique local addresses.
The reason why the L should be changed to 1 is because the block FC00 has not been defined yet. It has been proposed to be managed by an allocation authority, but this has not gained acceptance in the IETF.
I hope this has been helpful!
Laz