Article Toolbar - Container to Resize

I could not understand Container to Resize option inside the Article Toolbar field.

The default value is "flexicontent"

Container to hear also has "flexicontent" as the default value.

I would like to know if there is any other value that could be used with that option.

I would also like to know what it is used for!

That setting is for the CSS container surrounding the text to be adjusted or read.
The default container in the templates is "flexicontent".
Take a look at the item page structure with Firebug or some other tool.

So all text within that "flexicontent" container is adjusted with the text size button.

Usually there is no need to change this setting.
The only reason to change it is if you have a custom template where you have created your own container.