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At build time, %AgentsToRunOn% will contain every value you ticked on the queue dialog, in other words, every agent host name you specified. By selecting the Contains operator then specifying the expression $Agent.Hostname$, you're basically saying "Only run this stage when the list of agents I selected on the queue dialog contains the name of any agent registered with Continua".
Note: When explicitly forcing a stage to run on an agent, you bypass all the advantages of agent compatibility. If the agent you forced a stage to run on is unable to execute the stage then the build will sit on the queue waiting for a compatible agent whose name is the one you explicitly told the stage to run on. It will not under any circumstance pick an agent that's compatible whose name doesn't match the one or ones you specified.