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?](https://meta.stackexchange.com/questions/40724/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?](https://meta.stackexchange.com/questions/48417/should-we-zap-the-low-occurrence-tags/48418#48418), 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](https://superuser.com/questions/tagged/growl-notify) and saw it's been around for a while.

I apologize for my ignorance.