[Zope] custom_zodb.py

In the Zope Book v2.6, it says : "Each database connection maintains its own cache (see above, "Database Cache"), so bumping the number of connections up increases memory requirements. Only change this setting if you're sure you have the memory to spare. To change this setting, create a file called "custom_zodb.py" in your Zope installation directory. In this file, put the following code:" Am not sure which directory "Zope installation directory" pertains to. Is this the fresh directory from the Zope .tar.gz file ? Or is this the already-installed directory ? Or would it be the instance directory ? If so, which subdirectory do I place custom_zodb.py ? Regards, Danny