Hi,
I suppose you referenced this document:
In Neutron terminology segments appear on different layers, on the API a segment is a network type / seg. id / phys-net / net uuid tuple (see [1]).
What is interesting here that this segment has to be a representation on the compute where l2-agent (ovs-agent) can know which segment is the one it can bind ports.
That cfg option is in ml2_conf.ini, and bridge_mappings, where the admin/deployer can state which bridge (like br-ex) is connected to which provider network (out of Openstack's control).
So for example a sample config in ml_conf.ini like this:
bridge_mappings = public:br-ex,physnet1:br0
Means that on that compute VM ports can be bound which has a network segment like this: ( network_type: vlan, physical_network: physnet1, segmentation_id: 101, network_id: 1234-56..)
More computes can have the same bridge-physnet mapping, the deployer's responsibility is to have these connected to the same switch, whatever.