8 – What does it mean if expire is set to null a cache table?

I'm looking at the documentation for the field expires 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 element must
* not be deleted unless it is explicitly deleted.
* - A Unix timestamp: Indicates that the element will be considered invalid
* after this time, that is, it will not be returned by get () unless
* $ allow_invalid has been set to TRUE. When the item has expired, it can
* be permanently deleted by the garbage collector at any time.
* @param array $ tags

But when I look in a cache table, for example, cache_render I see that most of the entries are NULL in the "expire" column and I noticed that it only becomes clear when the cache is empty drush cr