The Summarize options are just shortcuts or pre-configured Custom options such as Reference Line, Band, or Distribution.

When using a Custom option, Tableau automatically opens a window to configure it. In this window, you can set the scope, the value, change the format, the aggregation, and configure many other options. Here's an example of the Add Reference Line, Band, or Box menu:

Another way to open this menu is with a right-click on an Axis and selecting Add Reference Line.

Custom options are not pre-configured, giving you the liberty to choose precisely what you want to display. The available values when editing a Reference Line, Band, or Box are the pills in the View and the Parameters. You will not find all the Measures in the Data Source. If you want to build a Reference Line, Band, or Box with a specific Measure, you need to add it somewhere in the View (usually in the Detail Mark Property, as it doesn't alter the visualization). 

In this chapter, the last section, Create a year-on-year comparator, includes a concrete usage of a Custom Reference Line. But before that, we need to take a look at the last Tableau element, Parameters.