This is all around applying meta data to your system.
Tagging allows you to add much more than the system tags such as $dimension or $measure.
Starting with those, $dimension is useful to tag your main dimension fields with. Not only does it add the meta data tag, it then will sort the list of fields in the chart wizard so that the $dimension fields are listed first. Similarly, in the Expression editor, $measure fields are listed first in the Field drop-down.
But you can add your own custom tags. These can be anything. For example, you could use System and Table names so that you know where fields have come from.
In the fields tab of the sheet properties, the tags will display when you hover over the field.
As of version 11, tags will export when you click the "Export Struture" button in the document's properties, "Tables" tab. You can then read this export into another QV document so, anything that might be useful to have in there could be added to tags.
