In this article, the meaning of _pen_ as in an enclosure, a cage, a tray, a box, a container [0]. A holding pen in this case means a holding tray, a holding box

In this article, the meaning of _pen_ as in an enclosure, a cage, a tray, a box, a container [0]. A holding pen in this case means a holding tray, a holding box, i.e., a container for holding things. A pen (cage, tray) for holding things. [0] https://www.thesaurus.com/browse/pen https://www.thesaurus.com/browse/pen