A Chain (what users see as a Plan in the UI) consists of one or more Stages and each Stage consists of one or more Jobs. Before progressing to the next Stage, every Job in the previous Stage must have completed successfully. There is no order to Jobs within a Stage so when a Stage is executed, all Jobs in that Stage are placed in the queue immediately.
All triggering strategies now work at the Chain level. Chains do not take Job information into account when deciding whether to trigger. If repository polling or repository trigger strategies are selected, only changes detected against the Chain's are used to determine if the build will be triggered.
All Jobs from a stage get put in the build Queue at the same time. Once in the queue Bamboo no longer has control over the job, until it gets results back from the agent.
Below is a diagram which shows Bamboo's build process flow, as well as the available plugin modules you can build.
Build Process For Chain
Build Process Per Job (Server)
Build Process For Stage
Build Process Per Job (Agent)