It is used to determine if the view cache files should be cleared immediately or if it ALL view caches should be expired.
The reason for this is if you have a large tree and change one of the top level items then the view cache for all the items must be clear. However if the number of subitems is large it would take a long time before all of them is cleared.
To remedy this we calculate a cache value, we multiply the number of subitems by the number viewmodes by the number of translations by the number of sitedesigns. If this number is larger then the threshold we only do the simple expiration.
