Java packages are funny things. Sometimes they are treated as a hierarchy based on names separated by ".", other times each package name is just a
string of characters and the "." means nothing special.
The "java -ea "x.y..." option is an example of the first case. It enables assertions in classes in package "x.y", package "x.y.z", and package "x.y.z.w".
Similarly, the classes in a package are placed in a directory tree based on the package names. In this sense, x.y is a subpackage of x.
On the other hand, "import x;" has no effect on classes in package x.y.
Go figure.
The unnamed package consists of all classes whose source programs don't start with a package statement. These classes don't have to be in any particular directory, but their directories must be in the CLASSPATH, which can include ".", the current working directory at the time of execution.
I think the term "named directory" means a directory whose name is in the classpath. This may also be the "current working directory", so the terms are not mutually exclusive. You can enable assertions in all classes in the current working directory which are in the unnamed package with "java -ea ...". There is no syntax to enable the classes in the unnamed package in a named directory.