A Class, in Photo Mechanic, is a color… one that has a meaning to me. Here's how my Class system works:
1 Private - Command-1 - Red - images I don't want anyone else to see, for whatever reason… think "honeymoon pictures"
2 Problem - Command-2 - Orange - images that would be useful, but need serious PhotoShop work for some reason
3 Polemical - Command-3 - Yellow - images that have story-telling value
4 Publishable - Command-4 - Green - general useful (publishable) images that don't fit into another category
5 Particulars - Command-5 - Blue - images that illustrate useful details
6 Personal - Command-6 - Purple - images that include people I know
7 Pattern - Command-7 - Grey - images of textures (wood, stone, concrete, grass, etc.)
8 Put Away - Command-8 - Brown - images I'll likely discard. Maybe they're not necessarily bad, but very similar to others, and that aren't quite as good. Whereas I discard blurry images instantly, I sometimes keep these (for now) because they're good, but just too similar
When you use Photo Mechanic for the first time, go to Preferences. You'll see the Classes in the middle of the General preferences window. Here, you can name your classes (Private, Problem, Polemical, etc.)
As with ratings, I make my first pass by double-clicking on the first image to bring up the Preview (large-screen image) window. The Command-keys noted above are the most convenient way (to me) of setting each image's class as I scroll through them. But as with star ratings, there's a Class window at the lower right corner of the image window if you'd prefer to set the Class by clicking there. As opposed to star ratings, I'm more comfortable with my Class settings on the first pass. I mean, an image either has someone I know shown prominently in the image (6 Personal (Purple)) or not.
Once you're done setting the Classes, move the images to the appropriate folders inside the "-5 Tag & Process" folder. It might be obvious, but if the image is 2 stars or less, move it to "2 & -" folder. If it's 3 stars or more, move it to the "3 & +" folder. And as noted earlier, you can delete the "-4 Set Classes" folder once it's empty if you like.
The next step is to tag images with keywords.