I am looking at the documentation for the expire field in the cache tables and the documentation in CacheBackendInterface.php says:

> @param int `$expire`  
> One of the following values:  
> - CacheBackendInterface::CACHE_PERMANENT: Indicates that the item should not be removed unless it is deleted explicitly.  
> - A Unix timestamp: Indicates that the item will be considered invalid after this time, i.e. it will not be returned by `get()` unless `$allow_invalid` has been set to `TRUE`. When the item has expired, it may be permanently deleted by the garbage collector at any time.

When I look in a cache table, for example *cache_render*, I see that many entries have NULL in the *expire* column and I noticed it only get clear on a complete cache clear, such as after running `drush cr`.

What does a null value set for the *expire* column in a cache table mean?