Configuring an IPv6 interface consists of assigning an IPv6 address to the interface.
Typically, you would use autoconfiguration to configure IPv6 on IP interfaces. Autoconfiguration assigns a link-local address and discovers prefixes and routers that are in use on the subnet. Interfaces with autoconfigured addresses also automatically request DHCPv6 address information.
If the interface is on the same link as a router that currently advertises an IPv6 prefix, the interface obtains that site prefix as part of its autoconfigured addresses.
The identifier portion of an autoconfigured IPv6 address is based on the MAC address of the interface's NIC card. Thus, autoconfiguration works best if the NIC cards are not often replaced. Otherwise, the MAC portion constantly changes as well. The instability would impact performance if other parts of the network infrastructure use stored IP addresses and therefore require stable addresses. In these cases, autoconfiguration might not be optimal.
As an alternative, you can use static addresses instead. Or, you can create IPv6 tokens as explained in Configuring an IPv6 Token. In both cases, the IPv6 addresses remain constant even if the interfaces are changed.