All right, this is a little bit embarrassing. What happens when a tag has zero questions?

I always thought that by removing questions from a tag, the tag will get purged. But, I was unsure about how often this occurs. I found there is a script that runs every 24 hours per the meta question What currently happens to zombie tags?.

But, what triggers a tag to be caught in this filter? According to the meta question Should We Zap The Low-Occurrence Tags?, a tag must be 6 months old and not have any questions associated with it.

However, I can't seem to find anything in help or on this meta or metaSE that explicitly states that this (both these conditions) is how the purge works, just a few threads on metaSE that describe different aspects of it at different points in time. Is there any official documentation on how this works, or am I just really terrible at searching?

Essentially, I see some tags with no questions that hang out for a while (probably the 6 months) at zero. I also wonder if a tag was created a year ago and then purged of questions, does it wait 6 months to be burninated or does it occur immediately? I ask because I recently purged the useless tag growl-notify and saw it's been around for a while.

I apologize for my ignorance.