I do not understand the usage of the "Custom CSS Prefix" in the post creation panel in the lower right-hand side. I typed in thisisTEST and expected to be able to see it embedded somewhere in the following location:

<div class="eb-mod-body">
アリゾナの広大な景色を目の当たりにして........
</div>
Perhaps as <div class="eb-mod-body_thisisTEST"> or as div class="eb-mod-body thisisTEST">
but a text search of the page did not turn up any copy of "thisisTest"

So I am lost as to what I should be expecting to achieve with "Custom CSS Prefix." Please enlighten me
Thanks,
Jim