Anycast IPv6 addresses are used for Anycast type of IPv6 network communication.
Anycast type of IPv6 addresses can be used only as destination addresses. Anycast type of IPv6 addresses are typically assigned only to IPv6 routers. Anycast addresses are from IPv6 unicast address range. So from a client perspective, it is not possible to determine the IPv6 address is an anycast address. The network devices which are dealing with Anycast type of IPv6 addresses are routers and only IPv6 routers are aware about the Anycast IPv6 address.
Click the below link to view the IETF RFC 4291 IPv6 address definitions.