This option lets you configure the number of days to keep transient end-systems in the database before they will be deleted as part of the nightly database cleanup task. The default value is 1 day. A value of 0 will disable the deletion of transient end-systems. Transient end-systems are end-systems that are Unregistered and have not been seen for the specified number of days. End-systems will not be deleted if they are part of an End-System group or there are MAC locks associated with them.
If you see the transient ES which you expect them to be removed but somehow it does not happen.
You have to check whether these ES mac is in any ES group