By default, you can send five test-script requests to any node. Although it is possible to change that configuration, let's see what happens when a node is already serving five requests, and you fire up another request for that node via the hub. The hub will keep polling the node until it gets a free test slot from the node. The test scripts are made to wait during this time. The hub says there are no free slots for the sixth session to be established with the same node. Meanwhile, on the node host, the node tries to create sessions for the five requests and starts executing the test scripts. 

Upon creating the sessions, five Chrome windows are launched and the test scripts are executed on them. After serving the first five test-script requests, the hub will establish the waiting sixth session with the node, and the sixth request will be served.