Nomad uses a gossip protocol to manage membership. This is provided through the use of the Serf library. The gossip protocol used by Serf is based on "SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol", with a few minor adaptations. There are more details about Serf's protocol here.
Advanced Topic! This page covers technical details of the internals of Nomad. You do not need to know these details to effectively operate and use Nomad. These details are documented here for those who wish to learn about them without having to go spelunking through the source code.
Nomad makes use of a single global WAN gossip pool that all servers participate in. Membership information provided by the gossip pool allows servers to perform cross region requests. The integrated failure detection allows Nomad to gracefully handle an entire region losing connectivity, or just a single server in a remote region. The gossip protocol is also used to detect servers in the same region to perform automatic clustering via the consensus protocol.
All of these features are provided by leveraging Serf. It is used as an embedded library to provide these features. From a user perspective, this is not important, since the abstraction should be masked by Nomad. It can be useful however as a developer to understand how this library is leveraged.