What is a "tag" in Spotfire?

Tags are annotations that can be attached to marked rows in a data table. Each row can only contain a single tag from each tag collection, but the document can contain many tag collections simultaneously. A tag collection is basically a column containing a set of different tags, or annotations. Tags can be used for filtering the data, creating lists, or transferring knowledge from one analysis to another. References: What are Tags?, How to Work with Tags