The Free On-line Dictionary of Computing (30 December 2018):
The form of FDDI interface where a device is connected to
both FDDI token-passing rings, so that uninterrupted operation
continues in the event of a failure of either of the rings.
All connections to the main FDDI rings are dual-attached.
Typically, a small number of critical infrastructure devices
such as routers and concentrators are dual-attached,
whereas host computers are normally single-attached or
dual-homed to a router or concentrator.
For example, a ring could be formed between a single router
and two concentrators (all dual-attached) then all other
components that need to be fault-tolerant (typically file
servers) can be dual-homed to both concentrators.