Comics use a numerical 1-10 scale. Actually a 0.5 to 10 scale for
the rare example where the interior is present, but the cover is
missing. A vague generalization on one of my comic book webshops
for grading http://www.theuncannyxmen.com/grading.html
The idea of slabbing originated as a means to preserve the opinion
of the third party grading arbitrator between buying and selling
parties. Once the item has been marketed, it's quite easy to "crack"
the slab open to read the comic. By cracking the slab open, you of
course have invalidated the grading. I have done this dozens of times
with comics that I've wanted to read. My main comic book focus is
"Tom and Jerry". I've opened every slab I've ever bought for that
line of books.
If you are flipping the comic for investment, or financial gain, you
will never crack one open. The high end of the market is very
touchy about restoration, and the slabbing folks are very very good
at spotting restoration.