Custom behaviour with custom fields?

I seem to remember someone posting a message about something they were
working on that took special custom fields and special things with them.
They used a unique prefix to the custom field name as a flag that the
field was special. PUBLIC_foobar would be a publically changable foobar
field or something. I’ve searched through the archives for the past few
hours, but with such a vague idea of what I’m looking for, I’m not having
any luck.
Does this sound familiar?
I’m mainly interesting in seeing how they did it and what they changed
before I dive in and start repeating work.