I suspect you are looking at a 39 ft sea ray made in the mid to late 1980’s where a wall (bulkhead) from salon to one stateroom (the one with bunks) has an approx. 3×4 sliding window. Nice feature to gain more seating area for salon. But who needs a stateroom with bunks unless you have kids.

It’s a “retractable bulkhead”, which is a marketing way of saying “very large watertight door”; in other words, a “very, very bad idea”.

If they stated “retractable bullhead” someone definitely is feeling “bullheaded” for it shoud be “bulkhead…”

Essentially it’s a sliding door or window in the cabin of the vessel. Manufacturers use this to “open up” the cabin area and make it look bigger or brighter.