XAO::DO::FS::Glue is a Perl module which acts like a glue that connects database with classes in XAO::FS.
my $odb=XAO::Objects->new(objname => 'FS::Glue',
dbh => $dbh);
A reference to the Glue object is what holds together all List and Hash objects in your objects database. This is the only place in API where you pass database handler.
It is quite possible that if XAO::OS would ever be implemented on top of some non-relational database layer the syntax of Glue's new() methow would change too.
In current implementation Glue also serves as a base class for both List and Hash classes and it provides some common methods. You should avoid calling them on Glue object (think of them as pure virtual methods in OO sense) and in fact you should avoid using glue object for anything but connecting to a database and retrieveing root node reference.
For XAO::Web case initialization of Glue and retrieveing of Global object is hidden from developer.
In theory Glue object should be split into ListGlue and HashGlue because now it mixes methods that know data structure inside List and Glue and this is not a Right Thing. But on the other side it is easier to keep everything that knows about SQL in just one place instead of spreading it over a couple of classes. So, do not ever rely on the fact that let's say _list_store_object is in Glue - it might move to some class of its own later.