Connection data
Connection data is a collection of opaque data that uniquely represents a player's connection to a Relay server. The primary purpose of connection data is to establish a communication channel between the host player and the joining players. Check out Connection flow.
A joining player receives the host player connection data from the Allocations service after requesting to join a session. After the player is bound to a Relay server, they can use the connection data to request permission to open a message channel with the host player. If the Allocations service accepts the request, the joining player can send messages through the Relay server to the host player.
To send messages through the Relay server, the message must have the allocation ID of the recipient. Players aren't aware of the allocation ID until they successfully request a connection to the Relay server and the host player.