hello nmerigot,
I think you have not beginning on the good base.
What we advise is to not manage database by yourself, but let SEBLOD do that for you.
In 99% of case, you just have to use the STANDARD storage for your field. You can have a look to this tutorial on the storage.
The FREE storage must be used in very special case; when it's not possible to do with STANDARD storage.
The tag you seen
<br />::catid_back:: , it's due to the storage CUSTOM, so you probably have a field with a bad storage.
What I advise in your case, it's to remove your custom table, and delete your content type with all fields, then start a new content type on the good way.
Also, don't hesitate to have a look on all manuals
Regards.
Lionel