Is it possible to filter a set of items based on some attribute?

Yes sure. Suppose that you have for example a biophisycal survey structured like cluster -> plot -> tree and you have an attribute "dbh" inside the "tree" entity, then you can select the trees that have a DBH greater than 20 with this expression: tree[dbh > 20].

Don't forget that the expressions will be evaluated considering the parent entity current schema node as "context", so the expression written before is valid if it's defined inside a attribute contained into the "plot" entity (the context will be "plot"). Instead, if you want to use it in an attribute defined inside the "tree" entity, the expression will become: parent()/tree[dbh > 20]. Thanks