Here's how I explain it to my students: a table without any overrides derives all of its formatting from the Table Designer. That's where you set the basic properties, ruling properties and the shading properties. I always advise my students to design their tables to rely exclusively on table tags created in the Table Designer—because its quick, and you can update all the tables in a document at one time.
But let's say you decide to change the ruling (or shading) for a couple of selected table cells—you do that in Custom Ruling and Shading, which overrides the settings from Table Designer. Later, if you decide the custom look was a bad idea, you reselect the same cells and choose From Table [Designer] for ruling and/or shading values and the table returns to the way it was before you added the overrides. None might work, and it might not—it depends on the table tag. From Table [Designer] always works.