There are times when you need to troubleshoot a build definition and why it is failing only on one server, or you bring a new build agent/server into the pool and want to test and need to send to specific build/server.
To Send Builds to a Specific Agent:
In the build definition, under General, add the Demand Agent.Name and set equal to the name of the build agent under the value column. This will ensure the build is serviced by only this agent.
To Send Builds to a Specific Server:
Same concept as above except that instead of targeting a specific agent name, you will set the Computer Name as the demand to send all builds to the server.
In the build definition, under General, add the Demand Agent.ComputerName and set equal to the name of the server under the value column.