In the content editor itself, you can create tables in your pages. You can right click and change properties for the whole table, the table row and the individual table cell.

One of the properties is class name.

Where on earth do these class names go? In the online demo version you can change the table cell to mceContentBody, and searching high and low for this, I find it as body.mceContentBody in editor.css.

For our users to change the class, should the classes be defined in editor.css, or if not, where?

Actually, I stuck it into editor.css and it worked!