This is an old revision of the document!
It's fast_cgi cache from nginx to never call the slow php web site. Just it cache the first time the returned page and return always the same. Useful for 95% of content because without login it never change.
It's total purge each time at 0h0min, and nginx don't keep more than 24h cache on 200 (normal page), 301/302 redirection, 404 (not found).
You can manually delete it via:
rm -Rf /tmp/nginx/*
It's enabled by default util your do POST request. Done via login, register, add to card into shop. When you do that's, it do CDIS cookie and disable the cache.