I'm not sure what this means. What do you mean by "the same way all of the other MC specific tags are stored"?
I mean it's written to the tag in such a way that only MC will understand what it means. It looks like it might be saved as a comment? Not sure. Other editors would probably be able to read the MC tags, but they probably wouldn't map to any kind of field.
What do people use this field for? What's it intended for? I've always been curious...
Access Rating is meant to prevent the kiddies from accessing any objectionable material that may be imported into your library.
So if you had porn imported into MC, and didn't want your kid to see it, you could set it's Access Rating tag to "Adult" or "R" or whatever, and then set Access Control to exclude any items tagged as being Adult or R. And it's password protected.
But since, Access Control isn't bound to JUST using the Access Rating field, I use it to filter out all kinds of things I want in my library but invisible. Like full album APE files, cover art, and yes... .
I also use it on my HTPC to filter out photo albums I don't want people to have to sift through on the HTPC when they're using it. For instance, I have desktop wallpapers, screenshots, etc. in my library, but on the HTPC that's all filtered out so all they see is my Christmas photos and things like that.