Until now, the tagging and labeling functionality was part of the Conditional Actions, which you can find in the Services menu. However, this has changed.
From now on, you can achieve the same effect by adding a dedicated Labeling step within your relevant processing pipeline.
With the Labeling step you can define a condition query in order to match documents and apply labels to them. The labels will be set in the documents’ meta data.
For details, refer to Configuring a pipeline.