You can easily see any table in the system. You can link tables together, append columns from linked table etc. However, if you need to group the data, easily see aggregations and load data from nested links, you would probably use some kind of query. And that's exactly what custom data source extension enables. Here, you can create a query to your data and use as data source for reports or widgets.
Let's say you have a table with thousands of orders
You do not want to load all of them to a report. Or you just want to create a simple widget with country and total amount. You need something like this:
As every extension, first you need to install that into the system. (see edit application)
In the table "Data Source" you add a new row. That will display a form with list of tables. In every table, you can select, which field (column) you are about to load. n case you aggregate the table, you can select the type of aggregation. The stored "query" can be used later in reports and widgets.
Input screen for the custom data source definition