classification systems other than Dewey and LC

I classify books by subject categories, such as SCRipture, HIStory, HAGagiography, etc. The capitalized letters are the top tier of the call number. The bottom tier is the first three letters of the author's last name. How/where do I add this info to my books on LT, and how can I sort the books by these call numbers?

If you want them sortable your choices are:

comments
tags (always has to be the first tag)
BCID (I don't use this one, so I'm not sure how many characters it sorts)

No one will stop you using the LCC or Dewey field, but it means that your call number could be suggested as an LC or Dewey call number on someone else's record, which isn't good.

If you didn't want to sort them, there is also a free text field, which you can find by hitting the change link under the Local field on the Edit screen.

What is BCID?

The BCID is the Book Crossing ID number for folks who use it. The rest of us are free to put other information there as it doesn't mess anyone else up.