Virginia Brilliant logically calls it: "the grill of the nuns' choir". p32

Charles Scribner III writes:
I cannot agree with Held's identification of the central dark area as an "actual tabernacle", for Rubens has clearly indicated a grill, the dimensions of which are sufficiently large (roughly 3 x 3.5m) to preclude any such possibility.
Moreover, there appears behind the bars a faint image of a seated or kneeling woman between curtains. The grill itself resembles the traditional kind that separates the cloistered nuns from the congregation. No such grill, however, is found today behind the chapel's high altar (or any other altar, for that matter).
The question of its precise identification - as either an actual or an illusionistic architerctural element - remains open and must await a detailed reconstruction of the chapel as it appeared in Ruben's time. The present chapel is the product of extensive remodeling during the intervening centuries. 2