Good question. I think that most of the time that's just an indication that the reply would only be of interest to the OP, and not to 'the public'. I've never heard it discussed, but I've thought of it as (more-or-less) a three-tier system:

• Unapproved nodes, read by regulars
• Approved nodes, read by regulars & searchers
• Front-paged nodes, read by regulars, searchers and browsers

At least that's how I've thought of it...