SLAAC

Stands for Stateless Address Auto-configuration

Hosts use the RS/RAA messages to learn the IPv6 prefix of the local link (i.e 2001:db8::/64), and then automatically generate an IPv6 address.

Using the command:

ipv6 address <prefix/prefix-length> eui-64

you need to manually enter the prefix.

Using the command:

ipv6 address autoconfig

you don't need to enter the prefix. The device uses NDP to learn the prefix used on the local link.

The device will use EUI-64 to generate the interface ID, or it will be randomly generated (depending on the device/maker).