The Number from Command variable type allows you to extract a single numeric value from a drawing commands that analyze data, including: Fit, Multivariable Fit, Box plot, and Histogram.
In the Number from Command variable, there are two dropdown boxes. In the From dropdown box, you specify a drawing command. In the Value dropdown box you specify what to extract.
The Value menu will change depending on what command is selected.
You can also use the gear menu on the right hand side of any drawing command to access the list of variables you can extract. In the example below, a variable is extracted from a Fit drawing command.
From that gear menu, you will see the Extract as Variable shortcut.
When you select a variable in this manner, the global variable is automatically created.