Labels (a.k.a. tags) are used for grouping multiple slaves into one logical group. Use spaces between each label. For instance, 'regression java6' will assign the labels 'regression' and 'java6' to the node. If you have multiple Windows slaves and you have jobs that require Windows, then you can configure all your Windows slaves to have the label 'windows', then associated the job with the 'windows' label. This allows your job to run on any of your Windows slaves but not anywhere else.