8 – What does it mean if a cache table is set to zero for the expiration time?

I look at the documentation for the Expire field in the cache tables and the documentation in CacheBackendInterface.php says:

        * @param int $ expires
* One of the following values:
* - CacheBackendInterface :: CACHE_PERMANENT: Indicates that the item should be used
* will not be removed unless it is explicitly deleted.
* - A Unix timestamp: Indicates that the item is considered invalid
* after this time, d. H. it will not be returned by get () unless
* $ allow_invalid has been set to TRUE. If the article has expired, it may be
* permanently deleted by the garbage collector at any time.
* @param array $ tags

But when I look into a cache table, for example cache_render I see that many of the Entires have zero in the "Expire" column, and I've found that only a full cache is deleted drush cr