Which IPv6 address type allows communication between subnets but is not routable on the public Internet?

Unique local addresses (ULA) are designed for local network communication within an organization, similar to IPv4 private addresses. They are routable between subnets within that organization but are not globally routable on the public internet. This provides private addressing for internal use. Link-local addresses are only for communication on a single link (subnet) and are not routable even between subnets. Multicast addresses are used for one-to-many communication, not for defining address scope. Global unicast addresses are globally routable on the public internet, making them the opposite of what the question describes.