This is a method of allowing the end-user of your app to "look up" a row(s) of data from your app's data source. You should use this method in one of two situations:
Setting this up is relatively easy. After you add a database as a data source to your app, simply click the 'Add app to database toolbar' toggle:
That's it.
Now, all you need to do is make use of the data being pulled into your app.
As soon as you click the 'Add app to database toolbar' toggle, you will have access to variables for each column in your database table. These new variables are labeled using the data source variable + column name:
You can use these new variables for anything you can use a normal variable for, such as creating document templates.
As long as your app is activated, it will appear on your database toolbar.
You can now select a row of data, then select the app you want to use that data in. You can even select multiple rows at once.