Addon reports enable you to set up fields in one report, then reuse those fields in a different report. Examples of why you would do this:
First, create the report you want to use as the addon report. In the example below, we've created a calculated field per sales order, and we've included the ID field of the sales order so that we have something to join the data to:
Next, we'll set up a new report. From the new report, click on Configuration -> Addon Reports:
We'll select the report we set up initially. Note that you can only use list reports as addon reports.
Next, we need to join the data for the addon report, so we'll do Configuration -> Customize Joins:
From here, we'll toggle over to customizing joins, and do Add Join:
We'll select our Sales Orders folder as the source, select our addon report from the bottom of the dropdown as the target, select the sales order ID field as the source field, and select our addon's sales order ID field as the target field. This connects the two ID fields so that data can match between the normal data and the addon report data:
Fields from the addon report will now show up in a folder named for the addon report at the bottom of the field list:
And from here, we can add and use the field just like any other field in our data source: