Some monitoring agent locations are capable of communicating using multiple versions of IP addresses including IPv4, IPv6 and 6to4. 6to4 allows IPv6 traffic to be routed across an IPv4 only network.
Some networks and equipment are capable of IPv6 native communication, meaning that they do not communicate using IPv4 at all.
IPv6-Only locations offer monitoring services aimed specifically at monitoring targets that respond to IPv6 addressing. While IPv6-only locations can be used to ensure IPv6 resolution is successful, the target does not have to be only IPv6 compliant. Rather, the IPv6-only monitoring locations are used to specifically monitor IPv6 resolution.
This is necessary because most networking devices that can resolve IPv6 addresses are also able to translate traffic between IPv6 and IPv4 using functions such as 6to4 tunneling or NAT64. The IPv6-only locations do not utilize any translation functions for the explicit reason of testing IPv6 accessibility without utilizing any translation.
Note that monitoring a target device using an IPv6-only location does not guarantee that your traffic will flow entirely across an IPv6 network, it merely indicates that your target was accessible from a starting node that could only send and receive traffic using IPv6.
Read the White Paper on IPv6 Monitoring to learn more about the history of IPv6.